E36 M3 your looking at expensive maintenance soon, plus not real fast. Upgradeable definitely, but then the mainentance thing kicks back in.
EVO/STI do look like out of the box rice, but you could pull the huge wing off, and give it more of a sleeper look.
S2000 is super fun, and reliable, and upgradeable, with a super charger preferably.
I drove the new Miata at the zoomzoomlive event, and it was fun but I think the S2000 is still a better choice. The miata had way too much plastic for interior, but that's common for mazda.
Supra would be cool, but old, and pig like, but can be big time fast.
RX-7 if you want to deal with Apex seal issues every 40k miles or less. Very fun car, I'm a little biased though.
350Z:bash:
You could look at older S4's but again it's a maintenance issue so pay now or later.
You could look at older 930's, but again maintenance. I've actually been looking at these, and you can find some in the mid to upper 20's.
It all comes down to what he wants. There is a ton of shiat out there. Good Luck.