I've got 720 different cars, and the 4 black second-hand race cars. There are still 9 cars I have to win (3 in driving missions and 6 in licenses).
There are 729 different cars in the game, i guess (at least in pal version). There are 730 in piggyback's bible, but two of them are the same one: Ford GT and Ford GT (no stripe). The first one can be bought and the second one must be won. But the only difference between them is colour. Both have the same name ("no stripe" is just in piggyback's list), same year (2005) and same mechanical features.