You must buy the following cars:
Callaway Sledgehammer Twin Turbo
Chevrolet Corvette ZR-1
Ford GT40 Mk1 (make sure you buy the Mk1, not the standard GT40)
McLaren F1 LM
RUF CTR "Yellow Bird"
Shelby Cobra GT500 (make sure you buy the Shelby GT500, not the Ford Shelby GT500)
TVR Cerbera Speed Twelve
The McLaren F1 LM is the second most expensive car in the game, so you will need a lot of credits to earn this achievement.