Need for Speed: Porsche Unleashed Hints for PC
Also on: Gameboy Advance
Click evolution and click usedcars, buy as much as you can afford but leaving around 20,000 credits. Then repair and sell the car(s) for about 200 credits more. You will see that you receive lots more credits and will be able to buy the best car for all the events.
Also on: Gameboy Advance, check out those pages for more help with this game.