If your talking about DTM cars, I don't think there are any in the game. For the German touring event I believe you can use any car made in Germany. Its pretty easy too. I used my BMW 135i I've souped up a little and it blew the competition away.
I used the Audi TT DTM car -- found it in used. I LOOOOOOOOVE it.
As to the person that said it can be any German car -- no, it cannot. That's the two Schwarzwald leagues, not the DTM.
Only DTM cars can be used in the DTM race series. The Lexus is probably the best 'deal' of the options. The Audi was over 800k, the Alfa Romeo about 1.5mil.
You can use the Lexus ISF racing concept, it's 750,000 from the Lexus dealership, as its a premium its available all the time. Can be tuned nicely and is fun to drive.
The M3 GTR is a beast, it won that series with no issues.I saw a BMW today, but I skipped it thinking it was worthless. Guess I should have read about it in the forums before passing it, oh well...