You do not need to buy a car for these. You can use the Toyota 88-CV you win from El Capitan. Do Not however whack a turbo 4 on it as it WILL chew the rears alot.
or for a challenge try a Mazda 787b for the Sarthe Enduro's
Best bet would be to check out Stock car race thread for some more idea's. You can take any of these cars that have won these events and buy a few parts to make it a bit more of a breeze through.