For $20k, you can go quite a few different routes as far as getting a car and making it fast. The srt4's biggest down fall is, it's FWD, I would really put a second thought into that. The good thing about it is it's cheap to mod.
At $20k though, you could make a 240 pretty nice and fast, get a used s2000, wrx, rx7, I would just try to consider all cars in the $20k price range and then test drive them, research them and then make your decision.