Mini Cooper 1.3, Daihatsu Mira bla bla, and the Suzuki Alto Works are the only cars that you can enter in the competition. Enter with the Mira fully modified (if possible) and leave with a Mini Cooper (the bonus prize i believe).
Yeah, I took my prize car Mira (look up to see where you can win it) and built it up a bit. I definitely didn't buy a car to run that in. That was fun racing!