That's one reason that its holding me back. There may be over 1000 cars in the game, but they are just the same cars with different years and parts.
One reason why I liked Forza was because you get to pick a car, customize it, and tune. In other words, make it mine. Whereas GT for PSP was to buy a car, no customization, and really limited tuning. I don't quite remember how GT5 Prologue was, but IIRC, its really similar to the PSP version.