Once you have unlocked "Perfectionist," and completing all of the other series, you will unlock the Ultimate World Championship. Finishing that will unlock the Super Secret League of Legends races. Completing that will unlock a gift car, a McLaren F1. From here, you will probably need another $2-2.5 million to purchase the rest of the cars. The quickest way to earn money is to keep replaying the "Notting Forest Challenge" in the World Series. A smooth run will take 1:05-1:10, and will net you $40,000. Keep on doing this until you have enough cash to buy all the cars. This will unlock on making your final purchase.