Have you considered miatas by any chance? They are not as small as you would expect on the inside. Plus the money you save in getting a miata (NA or NB) you can use to hook it up and you will have a very potent car.
If you wanna see some really nice miatas go to....
www.clubroadster.net
This is my favorite miata example......
MR-2 does not have a trunk that you can actually use.
S2000 aftermarket goodies are expensive plus the insurance is high.