Well, I'm building the M20 for boost because there's a whole lot less going on in the motor (Less cams, less valves, less valve springs, etc...) which means there's less to go wrong and won't cost near as much. And should it go wrong, getting another M20 is going to be a whole lot cheaper with lots of folk going to the M50 and E30's being parted out all over the place.
IMHO, keep it simple and have fun. Build up the motor now and get a few good months of motoring in. While you're having fun driving your car instead of working on it, obsessively plan your turbo setup and then do that build over the winter.
Or hell, if the motor runs good now (or ran good before your thermostat dealio), fix those issues and enjoy the car. Save your money, and do your upgrades over the winter.
Either way, take your time with it so it doesn't feel like work. That was the biggest mistake I made with my car. I was so focused on getting it done that I didn't take the time to enjoy what I was doing.