if you need the 4 seats i would get a E30 BMW and modify it to hell, theirs lots of very literate and technical sites for old BMWs, probably one of the few cars that you can almost be sure won't turn out to be a waste of money.
This is assuming you don't have the time or expertise for a old Alfa GTV, or don't want to follow the anime crowd and their ae86s.
The S13/14/15 is a fun car from what i've read, but its not super light, acceptable though
i still like the M5 idea personally! o jeez, i sound like a BMW fan hehe