Honest opinion from a guy who was looking at doing the SR20DET motor swap last year, stay with the M20 and boost it or go M50 (and later boost that).
The cost of a red top from an S13 + turbo upgrades to make it competent, the time and cash it'll take to fab up motor mounts, transmission mounts and the cost of a custom driveshaft and exhaust will put you probably many times over the purchase price of your car. To do the job right, I had about $4K budgeted and that was leaving the motor largely untouched internally.
If you go with the S14/S15 SR20, you have less to upgrade but then you're into OBD2 territory and will be picking up an aftermarket ECU + paying for a tune. Also, if you go with the S15, the 6-speed is FRAGILE and has a dual mass clutch/flywheel so you'll need to sort that out.
What got me to stop thinking about doing this swap? I could build a pretty reliable M20 that would handle boost very well (new valve springs, piston rings, HD rockers, good head gasket, ARP everything) for the money that I'd dump into an SR20 swap. Also if I were to sell the car, I'm not unloading a frankenstein project on someone which means potentially higher resale value.
I hear your frustration with the M20 and the rest of the mechanicals and stuck bolts, etc... But trust me, it gets better. Eventually you'll have loosened everything off at least once and you'll never have to worry about breaking stuff again (because you've put anti-seize on it!).
...These are the joys of owning a 20+ year old car. And hey, at least it's not an Alfa.
Edit: I'm not trying to discourage you at all because in the back of my mind I think it's a rad project to undertake, I'm just giving you the thoughts that made me stray away from the project.
Last edited by karmatose; 05-09-2012 at 01:21 PM.