I have the Porsche, Mustang, the professional gears and turbo, all the tyres, and Porsche Hubs. I'm not even going to bother buying the PT Cruiser... What's the purpose of the game once you get all 3 trophies and all the modifications?
Nothing, but like I said, when you get the trophies from the "Street Rally," they unlock the technical courses. After you've done those, you're done. Simple, but incredibly fun.