Just buy a rear wheel drive car, put it on 4 jack stands and take it completely apart. Weld in a roll cage, put in a seat and harnesses, replace every bushing in the suspension. Add brake lines and good pads, and do the wheel bearings while you're these. Buy some bargain, decent shocks/struts like Koni yellows. Sway bar (s) too. Get a bunch of wheels, and have a set w/ R comps like RA1s or NT01s, and a set w/ Z1 Star Specs for rain and transit miles. Speaking of which, trailer if you can. It's a lot easier and more comfortable, and when your motor don't go anymore it's a lot easier to get 250 miles back to DC from VIR. Beef up the cooling and oiling systems too. Have lots of spares and tools. Get lots of track time. Add power last.
For BMWs at the top of the track and autocross forum at bimmerforums there's a build thread subforum that's pretty good. Hint - when confronted by a 94 page thread - skip to the end and make sure it's not 94 pages of hi-res pics of all the deliveries of new parts and welding exercises...
"All my tools are hammers except screwdrivers which are chisels and punches"