Turbo Mopars, especially Omni GTH
http://www.turbo-mopar.com/
It looks kinda sorta like an old GOlf, but it's totally different. Can be practical, somewhat fuel effecient, and absurdly fast if you want. I couldn't find any for sale, but I've seen very clean ones with lots of go-fast work for under $6k.
Other than that:
2nd Gen Camaro
Nice big dumb simple American muscle. Pre smog so you have a free hand.
Early 70s 280z
Great styling, small foot print, good handling, simple, pre smog, Japanese reliability.
Mitsubishi GT (Naturally aspirated V6)
The twin turbos are cool but much more than $6k I think. Nice car. Lots of aftermarket parts if you wanna go rice with body kits and performance goodies.