Ultima GTR's are one of the best bang for the buck sportscars on the planet. There's a few catches though... you can get the low end version, and build one for under 15k, but you're using parts from a Ford Grenada. The high end version starts jumping in price pretty heavily.
Oh, and the windows don't roll down. At all.
Other than that, they're awesome cars... but not the fastest on the planet.
Here's the fastest on the planet:
Bugatti Veyron, 1001hp quad turbo W-16
