Well keeping it really budget.... Saxo VTS or 106 GTI (essentially the same car) - Loads of fun and cheap as chips, E36 325 or 328 - a few grand could see you in an M3 but I'd imagine there would be a fair old increase in running costs.....
Have a look on ebay and see whats being sold cheap to see what you fancy. I'd also look at the tracks you'll be driving, you'll know yourself what'll work on them and what won't.