Sadly, the Type IV cylinder heads aren't very well set up for turbocharging. They don't seal all that well against very high cylinder pressures, and the exhaust flow is not great unless you massage the heads. Which can get you into real trouble unless someone who knows what he's doing is the one doing the massaging. (You can kill flow by "hogging out" the ports; you can make the short-radius bend too tight and remove enough material that it cracks the first time you fire up the motor; you can remove enough material that the exhaust studs poke through, and so on.)
Stock-ish motors seem to be good up to about 7 PSI on normal gas, and 10 PSI on race gas. Not sure for how long, though--the head sealing issues can supposedly start at lower pressures than that!
Rebuilding a stock Type IV will run you $2K-$4K, if you do it right. You can frequently get away with spending less (sometimes a whole lot less!) but you start taking your chances with the engine's lifespan. (E.g., you really
do want to replace all of the valve seats. If you don't, your risk of having the seat drop is much higher.)
Rotaries require as much cooling as big V-8 motors. They're real heat monsters. Usually, those engines are good for one dose of overheating--and then throw it away.
You'll find that all of the "details" to cleanly install just about any $1K engine will quickly add up to the price of a nice Type IV; and if you want a very slick install the price will likely wind up in Six territory!!
No matter how much you spend, a Boxter will never be a 914. Nor will a 914 ever be a Boxter. The Box is a modern, civilized car. It is comfy, relatively powerful, fairly heavy, has stunning brakes, is quiet, handles very very well indeed. The 914 is a very raw driving experience. You sit on the ground in a flywheight chassis with modest power, it's not that comfy, it's loud and obnoxious, the brakes range from OK to sucky, but the handling is mind-blowing. (If you really want a Boxster, buy one--they're great cars, but they're not 914s.)
Megasquirt is very very much a DIY thing. Not only do you have to find/make the fuel delivery system and the air intake system, and not only do you have to come up with the fuel maps, but you even have to solder the ECU together!!
http://www.bgsoflex.com/megasquirt.html
Very neat stuff, if you're really into tinkering. Not in any way shape or form a "plug and play" system.
--DD