- Tell us about the road courses. Are they long and fast or short and technical?
- Do you want to modify it or leave it bone stock?
- If you plan to modify it, does your budget include mods?
- Do you plan to do all of your own wrenching or have someone else handle those duties?
- Do you have any power requirements? If so, quantify them for us.
I would expect the S2000 would likely be the most reliable option, but the least powerful.
I would think the BMW M roadsters will have the best weight-to-power ratios and, from most accounts, the most "raw" driving experience. I believe the Z3M Roadster used the older rear suspension design from the E36 chassis. I think finding a clean Z4M will be a challenge with that budget.
I would think the Boxster S would be a balance between the BMW and the Honda in handling and power.
If you are going to be tracking the car, you want to consider the cost of consumables like tires and brakes and do some research into their cooling systems to find out if the stock set up is adequate or if you have to budget for added or upgraded items like oil coolers or radiators. All three of the above have plenty of owners that track them, so you shouldn't have too hard a time finding resources to answer those questions.
I think I heard that
http://corner-carvers.com/ is a good signal:noise ratio site for a diverse group of road course enthusiasts.
