Keep using the same car for as many races as possible. A good idea is to take any car, and enter it in the lowest BHP series you can. (Put it on Easy just to help you earn more.) After every series, upgrade your car until it just falls short of the next series' restriction, and so on and so forth. A good car for this would be the 1970 Porsche 914/6. Not only can it be used for the first factory spec. championship (which is a decent earner), after entering it in the 150HP series, you can give it a surprising amount of power. Also, you can repeat races as much as you want, as the car level is determined by how much you win with that car.
To check how much more money you need to level up, when you see your ordinary level stats (the bar that fills up after your race to chart your level progress), press RT.