These cars become available options by downloading the Challenge Series and completing the corresponding task.

Unlock Battle Machine RX-7:
Win the first 30 Challenge Series Events.

Unlock Speed Machine 911GT2:
Win ALL 60 Challenge Series Events.

Unlock Dominator Corvette Z06:
Dominate over 30 Challenge Series Events.

At the 'Options' menu select 'Secret Codes' and input the following codes to unlock the corresponding bonus feature. These codes will only work once per profile.

Unlock $10,000:
Enter the code %%$3/"

Unlock NeedforSpeed.com Lotus Elise Bonus Car:
Enter the code-KJ3=E

Unlock Die-Cast Lexus IS F Bonus Car:
Enter the code0;5M2; (contains the number '0')

Unlock Die-Cast Nissan 240SX (S13) Bonus Car:
Enter the code?P:COL (contains the letter 'O')

Unlock Die-Cast Volkwagen R32 Bonus Car:
Enter the code!2ODBJ: (contains the letter 'O')