Those costumes must cost a bit. I would definitely go for a race between William Henry Harrison, James Buchanan, Rutherford Hayes, and Warren Harding.
As for Clinton, I think having him race Jefferson, Cleveland, and Harding would be great. Stage it at Good Guys.
The 4 costumes originally cost $50,000 in 2007. Since then they've had minor tune ups and patches for no more than $5,000 - $10,000 per season.