What races do you have left to win?
Formula GT is quite an undertaking, but if you win it, the Sauber C9 mercedes is yours!
I've found the jaguar xj220 race car is one of the best all rounders, as its got tons of grip and goes faster than 95% of cars.
The le mans bmw v12 is fasterthan the r8, and can see off the group C cars (sauber c9, nissan r92cp etc) which are probably the quickest cars in the game.
30k is not much to be getting on with, especially when you've got to buy the expensive cars to win the big races. I found the best car for tuning (all round, not total BHP, but driveable) is the Audi A4 touring car - you can get atound 800+ BHP from it, and my golly its quick.
EDIT: Bugger, thought you said 79%, not 7%. Disregard.
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Best early car is something like a lancer evo, or scooby. Or a second hand r34 skyline 4wd is excellent. Can win many of the early races with those.