For 500,000 you can murder the competition in nearly any race. For instance, with around 420,000 to spend, the RGT will lap Apricot Hill at least 4 seconds faster than the Denso Supra.
May I suggest you split the 500,000 in half and get 2 cars? That way you get more experience, more race coverage, and you don't murder the competition and ruin the fun of the game.
Get the RGT and the Tuscan, and upgrade each of them with race suspension, T3 tires, race transmission, and Stage 1 power upgrades. That should be good enough to do some damage.