Get all your licenses to gold if you haven’t already because the prize cars can be used for many races. The Miata you get for the B license is great for the early going. The RX-8 and the Fairlady Z are useful in early races too. The Vanquish and Viper are useful in some later races (not those with the race cars though). The Impreza can get you through the rally events just fine.
Doing the time trials in arcade mode works as well because you can cheese most of the races with the Panoz you win, or you can sell it for 500K.
If either of those options are too difficult for you, I recommend doing the championships again and again because you can win a car each time you win a championship, and the cars are useful. If you go this route, here’s a tip: if you’re leading a championship by 11 points or more going into the final race, save before moving on to the last race, then enter the race, then quit, then claim your car. If you don’t like the car you win, load the save back and repeat the process if necessary.
I know this is quite a mouthful, but it’s the best advice I have.