From GTPlanet Game Guides

There are only four prize cars available for GT5: Prologue Spec II, each being awarded to you for completing (to bronze or better) a class of races

C Class

Daihatsu OFC-1 '07

B Class

BMW Concept 1 series tii '07

A Class

Nissan Skyline Coupe Concept '07

S Class

GT-R Proto '05

A Class completion also unlocks Quick Tune Settings menu (for use in S Class and online races), the previously-hidden S Class events and the ending movie.