Nope. Some prize cars can be bought anyway, but those can be bought from day one.
If you want to win the 24 hour races the 'easy' way, just get a fast car (the Minolta Toyota 88C-V you win from one of the endurance races is a good choice), then leave the race on B-Spec mode overnight. Come back 24 hours later to find you've won. Not exactly as challenging, but not everyone has 24 hours time to kill.