If you're gonna do an engine conversion, then you don't need to worry with getting an E30 that was fast. Get anything that has a good body and the configuration you want (i.e. color combo, interior type, etc). The more you plan on upgrading, the less you have to concern yourself with upon initial buying.
Zionsville Autosport sells a CD-ROM based tutorial on how the swap is done and what parts are required. Additionally, they sell all the necessary parts (in a "kit" or seperately). Precision Performance Services (owned & operated by Pete McHenry) of Winston-Salem, NC was the first to perform a twin-cam E30 swap. Someone here will definitely have a telephone number with which to reach him.
E30M50.com documents one E30 owner's DIY conversion process.
Edit: Do a search of this forum for related articles. Lots of people have discussed the twin cam conversions lately.