My buddy just picked up a C5 Z06 that we are turning into a dedicated track rat. You would be surprized at how cheap you can pick one up for these days, If you are willing to do the work to gut it and upgrade the parts for the track you'll have a fast car with tons of available parts. Otherwise, I'd go over to the Miata or BMW camp. Rock solid reliable with tons of suport at the track.

Im with the fox body mustang recomendation... bodypanels are cheap and abundant.... its tried and tested.. a C5 is not a cheap aquaintance.. its almost cheaper to but a used GT1 corvette and just run that as is..