Here's three fully-tuned, race-modded road-going cars that took me 2 tries to win the GT300 championship, so you'll be tested with them.
Audi TT.
You'll have about 2400lbs./397hp. Understeers, but has 4WD, so you can control the power out of the corners.
Aston Martin DB7
Tt's quite heavy (3800lbs?) so it erases the power advantage (531hp) it has. The turbocharged (when tuned) V-12 sounds great, too.
Lexus IS200
Lacks power on the straights, handles beautifully. Should be able to make moves in the corners.
[R] Momo MR-2 GT or [R] Weds Sport Celica GT
Of course, you can't win these until you've dusted off the GT300 championship, and even then, it may take a few cracks at the title to win either one. Not as easy to win with as I imagined. Especially the Momo...don't make any mistakes!
Some easier ones:
[R] S2000 (road car) 364hp/2300lbs.
[R] RX-7 Inifini III '90
[R] RX-7 GT-Turbo '83 (good enough to win the GT500 series)