I'd argue that AWD is fine too, especially if rear biased. Any AWD AUDI or VW with the 1.7t or 2.7tt is pretty cheap- even the 3.0 turbo diesel is very tunable and sold w/ AWD. Mercedes and BMW both make RWD 4 cylinder turbos, modification abilities vary.

Personally, I think the best value in a RWD sports car is a first gen Porsche Boxster, and that is what I currently daily drive- you can get a nice used one under $10k, with the IMS bearing already upgraded. However, it's normally aspirated and not really tunable without swapping out the engine- however IMO it hardly needs more power- it will keep up on a track with many vehicles twice as powerful.

Old RWD turbo volvos are also underrated, you can tune a lot of power out of them for fairly cheap.

An unexpected thing for a lot of people is that diesels are often some of the most tunable engines- people get a lot of power out of German diesels. Because they don't have predetonation issues they can be a lot more reliable and cheaper to tune.