Take a look at what you've got and what races are coming up. If you're lacking, say a good mid-engined car or 4WD, go for one of those. Also, if there are any one-make races that you can't compete in yet, go for those. And, again, try out a few to see what you like.
Personally, though, I like the NSXs for around 90K. But that's just me; everyone has his/her favorites. Part of the fun, I think, is finding yours.