The Miata is never a bad idea. For the E36s, the M3s aren't that much more expensive, and you do get an LSD and a reinforced subframe, two things that you probably want for track use.

 

Lots Of good advice/suggestions in here. Another random angle nobody mentioned, your local bmwcca chapter probably holds their own events which means if u go BMW (any) it shouldn't be hard to find instructors at events that can really help you learn about the little ins and outs of your car. With that said, last time I went to one of their events, my instructor was there in his e46 m3 but had a race Evo at home and was really able to give some good info/advice on the car.

With that said I'd go with turbo Miata, Evo or a c5 vette but that's just me