Go race the Polyphony Digital Cup series race(10 races, 20 laps per race) in the Professional League (Im assuming you have the S licence) you can only use Non-tuned cars(ie no mods whatsoever) with the exception of upgrades tires. the best car to use is the Ruf CTR2 w/ racing super slick tires.. you only need to actually win 7 races to win the series just make sure you win the first 7.
the prize cars are random so save the game to your memory card on the 2nd to last race so you can go back and try again without having to do the series all over again if you dont get the car you want, if you have 2 mem cards you gan get both the Speed 12 and the F094/S which is in my opinion the best car to use, PERIOD!!!!! I used the F094/S to win the GT cup which is all F1/Cart cars and is probably the hardest race series on the game (10 races, 40 laps per race)