Nissan GTR (as Nuck said); FREE when earning gold in the S licence.
-You can't custom paint that one, if you do it'll run you 77k for the '07, 157k for the '09 SpecV.
Chevrolet Corvette ZR1; Racing Modification makes this a beast; total cost approaches 400k.--- You could also get multiple Ferrari's and Lambos' too.
Mazda Furai; Free when earning gold: NR-A Roadster cup in B-Spec.
Pagani Zonda R; Free when earning gold: GT World Championship in B-Spec.