Alright then:
I used my fully upgraded TT to win the championship race, and it works well in the next level of races too.
I would suggest, to earn more money, that you run the (Beginner) Tourist Trophy series a few times and sell the car you win. Same thing with the Beetle series (but TT earns you more) and make sure your TT is fully upgraded, then you can usually smoke the GT Champ series a few times and get even MORE money.
My best/favorite cars in GT3 were (all full upgrades, and in no particular order)
Audi TT (4WD)
Mazda Miata
Lancer Evo VI TME Stripe J (4WD) (this will win every Rally Race there is)
Nissan Skyline R34 (or R32 - I'll have to check and get back to you, it's black with just a few stickers up by the front tire)
Tuscan Speed 6 (FR)
Acura NSX (best you can buy) (MR)
Those were for regular races, then I started winning things like the Escudo, F1's and all the other supercars...and that is by no means a complete list
Can't remember which FF I used off the top of my head though, had a few beers tonight
But still, if you have tuning problems (since your about to enter the world of tire damage soon) - go to:
www.gtvault.com - that should really help you out a lot.
UMP