Well, what are your intentions? Do you want to play A-Spec, B-Spec, or online?
If you want to play A and / or B-Spec, use prize cars to advance as far as you can without spending credits on anything, until you're required to do so. The two modes bounce off of each other nicely as far as "win car from license and / or special event, use car in A-Spec, win race event, win another car, use car in B-Spec, win race event, win yet another car, rinse and repeat".
If you want to race online, give spectator mode a try to see what some of the other drivers are using. If you see something that catches your eye, try buying that particular car for yourself. Find out if you like it showroom stock before dumping your hard-earned credits into upgrading it.
Good luck!
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