It depends on if you are asking about cars that have the EXACT specification AND livery as they have had when they raced the ADAC N24 in real life. In that case I can only remember the 2 premium GT-Rs, and the Falken Skyline, the ones you mentioned. Someone correct me if I'm wrong.
There are however a bunch of race cars that have raced in the real N24, but are not featured in the same spec and livery as when they raced the N24 in real life. For example R8 LMS, M3 GTR, Viper GTS-R, Astra V8, MB 190E and probably more that I don't know of or can think of right now.
And of course a lot of cars that have been used as race cars in the real N24 are featured in GT5 but only as road legal cars.