On a PC you need to use a savegame editor (do a web search) to change an existing car to the Koenigsegg CCX (buy a cheap car in career mode, then change it to CCX using savegame editor). It's available in quick race, but if you add it to your career garage you can upgrade the parts to ulimate. I uploaded a video to youtube using the CCX at East Ocean Expressway:
> There Is Buggati Veyron Also Available But Its Locked.
The Veyron is a standard tier 1 car in the PC version of Undercover. Although it has good acceleration and top speed, it doesn't turn well (in Undercover). It's only good on high speed tracks. The Z06 is a better all around car. The Zonda corners the best of the tier 1 cars, and the F1 is somewhere in between. The CCX is not a standard car, but it is the best overall tier 1 car.