Complete career mode, then win any quick race drift with the red Audi, then win any quick race sprint with the silver Audi.
When you have done this unlock all the Poliec cars (Civic Cruser, Inerseptor and the SUV) with reward cards.
Go onto career to the car lot and buy the Audi.
WARNING: save the game afterward. The only bad thing is you cannot put performance parts on the car.