One thing I find is that odd levels (the ones that don't end in a 2 or 7) will probably have more recipes available on the market, and possibly also for better prices. Also, lower level characters like your Brute at 12 tend to have less competition looking to frankenslot. In the early levels many players don't seem to bother much with slotting anything beyond TOs/DOs/SOs. So those are likely to be cheaper ways to experiment.
On the other hand, the "upgrade" levels are going to be more competitive in terms of buying from the market. Those are the levels where you can first slot the next grade of enhancement. For example at 12 you can slot a level 15, at 17 you can slot a level 20, and so on. Also some desireable sets open up at particular levels like 20 or 35. Many folks see levels like that as a perfect opportunity to re-examine all of their enhancements, since they're upgrading anyway.
Something to remember when you start planning around sets is "The Rule of Five". You can only have five of the exact same set bonus. You could have 5 that grant you 9% accuracy, and 5 more that grant 7% accuracy, and 5 more that grant 11% accuracy, and they will all get along together very well. But adding in a 6th of either one is pointless as it will just get ignored. (Mind you, this is referring to the game's internal name for the bonus, so there are some quirky exceptions if the bonuses come from different sources. You can find the exact name of any bonuses you have already slotted by opening Combat Attributes, in your Powers window.) Also, for those who are unsure, set bonuses apply across your entire build, not just to the power they're slotted in.
While we're on the topic... you seem to know what "Frankenslotting" refers to, but for those who don't I'll give a brief example. Let's say you have two slots in a power and you've put in two level 50 common IOs, one accuracy and one damage. That power is currently getting 42.4% to accuracy and 42.4% to damage. Now look at a level 50 Acc/Dmg piece from a set. That one enhancement gives you 26.5% to accuracy and 26.5% to damage. So if you can find an Acc/Dmg from two different sets that the power can accept, you can slot them both for a combined total of 53% accuracy and 53% damage. Still only using two slots. That allows you to get the same boosts using fewer slots, or to get more or higher boosts from the same number of slots. Like Frankenstein's monster, you're building something big from a bunch of smaller parts.
I'm no expert, but that's what my experience has taught me. Have fun experimenting 
You don't have to go all Sets or all frankenslots.
How much Inf are you budgeting for this? Unless price is no option, you'll be limited by what you can afford. And, of course, the availability of the particular items at the Market. That's what guided me when I started.
If you're going for Sets, in my opinion, the attack and armor powers are where you should start. Almost every type of character will have 3 or more of these powers which will take the same kinds of enhancements. So if you use the same Sets, you'll have a good start on stacking the set bonuses. After that, if you have another power that will take a Set which includes a set bonus you want to add to, then you can consider it and weigh that set bonus's benefit relative to maximizing the power's attributes with frankenslotting. This is the mini-game that can keep you at the Market or fiddling with a build planner for hours.
Teams are the number one killer of soloists.
Yeah I know I don't have to go all one or all the other, this is more of an expirement as I mess around and decided I want to build one more focused on trying to Frankenslot to squeeze as much as I can into each ability, if I pick up a set bonus 'shrug' but its not the focus. The later project will be more about hitting the sets for the extra bonuses hard.
Not setting a budget yet, but I will be doing a lot of market hunting for deals to try and bring in the best deals. will do alignment/vanguard/reward merit grinding and ae tickets to bring in other stuff as I start to play around.
If you pick the Rad/Rad, you will find that a lot of your Defender powers will use things that are not popular, therefore cheap, therefore excellent for Frankenslotting. Your blasts, well, half of everyone [as mentioned] has three blasts. I recommend bidding on three levels at a time, assuming you're going to check every couple of days. For similar reasons, I recommend trying to only do a couple of powers at a time. That's around ten slots; you can hold ten IO's in your tray and you will probably be able to get four to eight slots filled near-immediately, so you can put out a spread of bids on the remaining items.
Mini-guides: Force Field Defenders, Blasters, Market Self-Defense, Frankenslotting.
So you think you're a hero, huh.
@Boltcutter in game.
Well after my 5 years of ingame time I finally broke down and tried common IO's on a character. Got my Wp/DB/energy tank set up with common's and it is good.
My next attempt is going to be a 'frankenslot' project, and from there the project after that will be a full 'set based' setup. My problem is I don't really know which toons would be easier to do 'frankenslot' vs. full sets.
The toons I'm deciding between are:
Rad/Rad Def-currently 38
DM/FA Brute-currently 12
Nightwidow/fort(dual build)-currently 37
Elec/nrg/elec blaster-50
Elec/rad controller-currently 40
Mainly looking for insight from people who have done both approaches. Not looking for builds or 'go check Mids' comments. Just which ones seem more feasible for either approach and why.