Don't bother.
You can upgrade the vehicles individual parts with credits you earned by winning race and signing sponsors.
A very large downside to this game is the out-of-bounds reset has a mind of its own. Some tracks you can go 5 feet off the beaten track and go back on 20 feet down the track. But other tracks if you slid off like 1 foot and go right back on it will still reset you. This is so annoying when you are in first in an expert race.
The bottom line is if you want a reason to break your psp then buy this game, because you will get mad at it.