Even if you're going for pure team support and not for soloing AVs, building for +recharge and perma-PA is still better than building for ranged defense.
Here's an apparently well-kept secret: ill/rad doesn't need an uber purpled-out build to solo AVs or be an extremely survivable asset to a team. You can do either of those things on an SO build--but not as effectively as you could with a perma-PA build. If you're going to IO your ill/rad out, though, +recharge is still the way to go.
Of course, build your ill/rad however you like. If you must go for defense, the amount you'll need depends on the foes you're fighting. With RI fully slotted for -ToHit, a mere 6% defense will softcap you against even-cons caught within the debuff. Higher level foes will require more defense to floor their chances to hit you as their level scales up relative to yours; and level 50 AVs with their debuff resistance will require 39% defense to floor their chances to hit you, even with RI fully slotted for -ToHit.
Yet I urge you to reconsider building for +recharge. Remember that with enough recharge, you will provide teams with:
1) omnipresent indestructible tanks
2) perma-AM
3) crippling debuffs that recharge so fast you needn't sweat the anchors being killed
4) those omnipresent tanks again
5) a fast-pulsing heal
6) AoE holds that recharge hella fast
7) great damage output with a tight attack chain, and
8) omni. present. tanks.
Comparatively speaking, a defense build would offer a team less of each of those things, with little or no additional survivability.
There are precious few builds in CoX that are better if you don't build for defense. But omg, ill/rad is way up at the top of that very short list. Plus like I say, there's no real disconnect between a perma-PA AV soloing build and a team-support build, when it comes to ill/rad. Power selection early in the game may be different, and slotting priorities may differ, but in the end, the perma-PA build is the way to go for either purpose.
EDIT: originally I slipped you a peek at my own Ill/Rad build, intended to show you how a lot of +recharge could be had fairly cheaply by level 42, perma-PA could be attained by level 48, and all this without any purple or pvp IOs. But now I've thought better of it. Your focus is very different from mine; you'd want to take the force multipliers RI and EF earlier than I did, you'd want Mutation which I skipped, and so on.
Still, I stand by my assertion that +recharge and not +defense is the best way to go, even for a support-focused build.
I agree with Chaos String's comments that Recharge is more important than Defense. An Ill/Rad's defenses are based upon distraction and debuffing -- if those are working, then the baddies won't even look your way. You don't need to go for a max build to be very effective, but faster recharging powers is the best benefit for an Ill/Rad.
The order of your power choices are a problem. First, Phantom Army is the most important power in the set, and should be taken immediately at 18. Certainly it is far more important than Recall Friend. Recall is nice to have, but not essential. If you really want Recall Friend for those lower levels, do what I did . . . go with Teleport as your travel power. You can always change it later if you want. My Ill/Rad leveled up with Teleport; when I changed it to fly later, it just didn't feel right, so I went back to teleport. There actually are some advantages to teleport, especially when combined with Hover.
Spectral Terror is your best (and in your build, your only) AoE control power. It should be taken at 26, not 47.
I am a strong advocate for taking Deceive early, as it is an amazingly effective control power in low levels. I suggest taking it at 4 or 6. My Ill/Rad guide explains why.
Your build needs Hasten, taken either before PA or soon after. Ling Rad and even EF can wait.
Reconsider your APP choice and powers. I explain in my Ill/Rad guide why having a fast single target blast is very helpful, so I recommend you get one. Even on a team-oriented Ill/Rad, the ability to take down single foes will be one of your better contributions to the team. I explain in my guide why I prefer Fire and Ice, but Primal certainly can be used. However, the main benefit of Primal is Power Boost, to be used on one of your AoE controls like EM Pulse . . . which you didn't take. Conserve Power is nice, but I made up for it using Consume instead.
My opinion is that control powers are better than Leadership -- I think control powers add more to the team. So, I would drop Leadership to add Flash, EM Pulse and the single target Blast.
LOCAL MAN! The most famous hero of all. There are more newspaper stories about me than anyone else. "Local Man wins Medal of Honor." "Local Man opens Animal Shelter." "Local Man Charged with..." (Um, forget about that one.)
Guide Links: Earth/Rad Guide, Illusion/Rad Guide, Electric Control
Although I am a nub, I too will throw my 2 cents in if I may.
I am of the mindset that in a team dynamic, the best thing each individual can do to contribute to the success of the team is to play to their strengths.
The need for specific strengths in teammates becomes more apparent as the increase in difficulty rises.
Certain ATs/power set combos excel at different things. While all can contribute to a team, to varying degrees, in one way or another there are certain AT/power set combos that excel at specific tasks.
My advice would be to build your AT/power set combo to enhance it's strengths.
An Ill/Rad, or any controller, in an ideal team setting does not need ranged defense as much as need recharge to get those extremely useful "control" powers back up and running asap.
For the record, in an ideal team you have an individual such as a Tank, drawing aggro to themselves and away from those playing a support role.
I will say as all others do..do whatever you want to do that is fun for you. You're paying a monthly to have fun in the manner you want.
Thanks for the replies folks.
See what I can do then to gut some other characters for funds then, because it won't be cheap.
Local, if I'm reading one of your builds correctly, global recharge of 106.3% is enough to get perma-PA/AM/hasten?
It really depends on what you call "Perma-PA." If you take into account the three second cast time and despawn time, then you can get by with less. I based my numbers on getting enough recharge to get under 60 seconds, but you may be satisfied with under 63 seconds.
But yes, you get 95% from fully slotting PA for recharge, 70% from Hasten and 30% from AM. So you need about 105% . . . but on Mids, the numbers seemed to need slightly more than that to get under 60 seconds.
LOCAL MAN! The most famous hero of all. There are more newspaper stories about me than anyone else. "Local Man wins Medal of Honor." "Local Man opens Animal Shelter." "Local Man Charged with..." (Um, forget about that one.)
Guide Links: Earth/Rad Guide, Illusion/Rad Guide, Electric Control
Thanks for sharing your build. Here are some scattered comments:
- I agree with the consensus to scrap Defense and plan for Recharge. The great thing about doing this is that you will begin to feel the effects as soon as you start IOing. With Defense builds you don't see much benefit until the build is nearly complete.
- I agree that Spectral Terror and Phantom Army need to be top priority. Pick them the moment they become available.
- Obligatory comment about Fly: Unless you're taking it for concept purposes, I'd nix it and use a jet pack instead. You can even opt to skip a travel power altogether and use Ninja Run from the Ninja pack. I do this on at least half my characters these days. AM is going to be increasing your run speed anyway.
- Deceive needs to be taken early. This power is tied with Mind Control's Confuse for the longest duration single-target mezz in the game, AND you can use it without drawing aggro. Use it, love it, laugh at the Controllers who don't have it.
- You are probably going to want a single target blast and AoE blast of some kind. Fire is a particularly good choice because the heavy damage it does lets you kill things before they can heal back the illusionary damage you deal.
- You want Hasten. Some people hate this power. I used to. Then I set it on auto. It makes an absurd difference in the performance of Controllers (and everyone else), to the point of being overpowered. Might as well leverage it.
- There actually IS a good reason to take Recall Friend early: if you will be doing lots of exemping for Task Forces. You've got invisibility, so you'll end up being the team stealther. But don't take it over Phantom Army.
Hope that's helpful. 
Going a somewhat different route with my ill/rad since I have no particular interest in the uber builds for soloing AV's. My general intention was to maximize recovery and (ranged?) defences, and be a survivable asset to a team. AM+PA up most of the time would be nice though.
So far, I have something like this planned out (current build is half SOs). Not sure what kind of +def I should be shooting for, as radiation infection tends to help substitute for a lot of this (I think, not much of a number cruncher).
Hero Plan by Mids' Hero Designer 1.401
http://www.cohplanner.com/
Click this DataLink to open the build!
Void Spectre: Level 50 Magic Controller
Primary Power Set: Illusion Control
Secondary Power Set: Radiation Emission
Power Pool: Flight
Power Pool: Fitness
Power Pool: Teleportation
Power Pool: Leadership
Ancillary Pool: Primal Forces Mastery
Hero Profile:
Level 1: Blind -- Lock-Acc/Hold(A), Lock-Acc/Rchg(3), Lock-Rchg/Hold(3), Lock-EndRdx/Rchg/Hold(5), Lock-Acc/EndRdx/Rchg/Hold(5), Lock-%Hold(7)
Level 1: Radiant Aura -- Dct'dW-Heal/EndRdx(A), Dct'dW-Heal/Rchg(7), Dct'dW-Heal/EndRdx/Rchg(9), Dct'dW-Heal(9), Dct'dW-Rchg(11)
Level 2: Radiation Infection -- HO:Enzym(A), HO:Enzym(11), HO:Enzym(13)
Level 4: Spectral Wounds -- Thundr-Acc/Dmg(A), Thundr-Dmg/EndRdx(13), Thundr-Dmg/Rchg(15), Thundr-Acc/Dmg/Rchg(15), Thundr-Acc/Dmg/EndRdx(43), Thundr-Dmg/EndRdx/Rchg(46)
Level 6: Accelerate Metabolism -- Efficacy-EndMod(A), Efficacy-EndMod/Rchg(17), Efficacy-EndMod/Acc/Rchg(17), Efficacy-Acc/Rchg(19), Efficacy-EndMod/Acc(19), Efficacy-EndMod/EndRdx(21)
Level 8: Superior Invisibility -- LkGmblr-Def/EndRdx(A), LkGmblr-Def/EndRdx/Rchg(21), LkGmblr-Def(23), LkGmblr-Rchg+(23), HO:Cyto(46), Empty(46)
Level 10: Hover -- Flight-I(A)
Level 12: Swift -- Flight-I(A)
Level 14: Fly -- Srng-Fly(A), Srng-EndRdx(25), Srng-EndRdx/Fly(25)
Level 16: Health -- Mrcl-Rcvry+(A)
Level 18: Recall Friend -- RechRdx-I(A)
Level 20: Stamina -- P'Shift-EndMod/Rchg(A), P'Shift-EndMod/Acc(27), P'Shift-EndMod(27), P'Shift-End%(29)
Level 22: Phantom Army -- ExRmnt-Acc/Rchg(A), ExRmnt-Acc/Dmg(29), ExRmnt-Acc/Dmg/Rchg(31), ExRmnt-EndRdx/Dmg/Rchg(31), ExRmnt-Dmg/EndRdx(31)
Level 24: Enervating Field -- EndRdx-I(A), EndRdx-I(33)
Level 26: Lingering Radiation -- TmpRdns-Acc/Slow(A), TmpRdns-Acc/EndRdx(33), TmpRdns-Rng/Slow(33), TmpRdns-EndRdx/Rchg/Slow(34), TmpRdns-Acc/Dmg/Slow(34)
Level 28: Deceive -- CoPers-Conf(A), CoPers-Conf/Rchg(34), CoPers-Acc/Conf/Rchg(36), CoPers-Acc/Rchg(36), CoPers-Conf/EndRdx(36), CoPers-Conf%(37)
Level 30: Maneuvers -- LkGmblr-Def/EndRdx(A), LkGmblr-Def/EndRdx/Rchg(37), LkGmblr-Def(37), LkGmblr-Rchg+(39)
Level 32: Phantasm -- ExRmnt-Acc/Rchg(A), ExRmnt-Acc/Dmg(39), ExRmnt-Dmg/EndRdx(39), ExRmnt-+Res(Pets)(40), ExRmnt-Acc/Dmg/Rchg(40)
Level 35: Mutation -- RechRdx-I(A)
Level 38: Tactics -- GSFC-ToHit(A), GSFC-ToHit/Rchg(40), GSFC-ToHit/Rchg/EndRdx(42), GSFC-Rchg/EndRdx(42), GSFC-ToHit/EndRdx(42), GSFC-Build%(50)
Level 41: Conserve Power -- RechRdx-I(A), RechRdx-I(43), RechRdx-I(43)
Level 44: Temp Invulnerability -- TtmC'tng-ResDam/EndRdx(A), TtmC'tng-ResDam/EndRdx/Rchg(45), TtmC'tng-ResDam(45), Aegis-Psi/Status(45), HO:Ribo(50)
Level 47: Spectral Terror -- Abys-Acc/Rchg(A), Abys-EndRdx/Fear(48), Abys-Acc/EndRdx(48), Abys-Acc/Fear/Rchg(48), Abys-Dam%(50)
Level 49: Vengeance -- LkGmblr-Rchg+(A)
------------
Level 1: Brawl -- Empty(A)
Level 1: Sprint -- Empty(A)
Level 2: Rest -- Empty(A)
Level 1: Containment