I played though the game and not had to actually BUY one single car for this cheevo, it might be bc I got the car for at least doing 30 races off the challenge series.......
Edit: I didn't have to buy any cars at all, and the 240 SX (car I received by finishing 30 races in the Challenge Series) was number 6, or didn't count
1) First race picked the Lotus Elise (T4)
2) got the Audi TT (T3)
3) picked the Toyota Supra (for no good reason) (T3)
4) picked the Lambo Gallardo (T2)
5) unlocked cheevo when I was given the BMW M6 (T2) after/during the mission "Meet Carmen"
As you can see, the game GIVES you 5 cars, so save your money for a good Tier 1 car and the "ultimate parts"