Beat the 14th boss to unlock his car, which is the Porsche Carrera. You will need to buy it from the shop.

When you beat Razor in chapter 15 you will unlock the BMW M3.

You can repeat any challenge you have already won to get extra money. You might even be able to get heat metal and win more money.

When you can get the Mustang GT, Mazda RX-8, Audi TT 3.2 Quattro, and Subaru Imoreza WRX STI, choose the Audi TT 3.2 Quattro. It is better then all the rest, unless you can afford the Subaru.

The easiest and quickest way to get money is to go back and beat the ford mustang gt you get 1500 points for modifacation upgrades.