If you get an M3, dont expect to be able to build the bottom end and turbo it for a reasonable cost.
You can boost it and get good power without building the bottom end, see Chelsea Denofa's E34. Thats a pretty stock engine (HG I think, and injectors of course) but it makes plenty power. No idea specifically the mods, but I've seen the engine and it looked like the engine came stock with a turbo. Some random cast log style manifold with a decent snail and it ripped. Look into turboing other beemers as there are cheaper ways to go about a turbo 3 series and get turbo power. You wont get as much as a turbo E52, but youll make more than it ever will NA unless you drop lots of paper on someones desk. I'm sure an M3 turbo getup would be more than the setup he mentioned to me the one time I spoke to him.
http://www.youtube.com/watch?v=gCCSqH5Pzi4&feature=fvw
Im sure you can build that for around 10-11gs. And thats not an M3 motor.