I know your first car has to be 10K or less (excluding credit transfer from GT3) but what I did was buy the cheapest car with the best performance. I bought the FC3S RX-7 GT-X from Historic car lot for like 5.5K. Then I raced it for a while with easy races (Sunday Cup, FR Cup, etc..) until I had about 31.5K or enough to buy the HPA Stage II R32 from Tuner car lot. This car is by FAR the best value performance car in the game. It has like 500 HP and like 600 Torque with all wheel drive. This car was able to win me Professional events like Clubman Cup, Turbo Cup, and Tuner Cup with ease. By then I had means to earn enough money to start buying the top cars in the game.