Most of the good ones are cheap, since none of them are new.
My list of great sims:
GTR (FIA GT 2003 Season)
GTR2 (FIA GT 2003-2004)
rFactor (Nothing at all almost, but tons - literally - tons of free downloadable content, and the sim with the best mod groups)
GT Legends (You have that one, no?)
Race (2006 WTCC season + other classes)
Race 07 (2006-2007 WTCC, Minis, F3000, Caterhams, Radicals, Formula BMW)
Look for GTR2 (you will find this one very cheap), rFactor (around 40$), Race 07 (40$ on steam).
The best bet is rFactor, though. You will find almost any car for it, just have a look here:
http://www.rfactorcentral.com/index.cfm
And this site is great for keeping up with the mod news:
http://www.virtualr.net/