I used a '69 Chevrolet Camaro I bought at the Chevrolet dealer. It is one of the few cars that can be race-modded at GT Auto. This gives you the advantage of having adjustable downforce, something which helps the handling greatly and is more or less a necessity when facing a pack of race cars. I also tuned it to the maximum including soft racing tires, though it later turned out that the supercharger is expendable since it does not add any more power at higher revs.
Anyway, the total cost of this project should be somewhere between 300,000 and 400,000 credits including the car, and it enabled me to win every race of this event quite easily. It handles very well, is pretty much equal to the fastest cars in the pack in terms of raw acceleration and declasses the AI under braking and in the bends.