I just picked up my PS2 and GT3 (of course) so, I'm not very far into the game either. But I've have'nt had to make that desision yet.
I started with the Miata, before I had to decide on what next I won the Mitsu Evo which opened up other races, then I won an R32 Skyline, then the proto Skyline then the Gillet. I'm winning cars faster than I can tune'em.
Anywho, just keep racing and you'll probably win what you're look'in for. If your patiance can hold up that is. I've won about 4 or 5 lancers, and skylines each, I sell the extras unless they're a significant up grade to what I have already.