That was a nice way to lead me in the wrong direction.
I guess it was not intended, but in Germany the actress has a five letter Christian name followed by a "von" instead of a "de".
The movie we are looking for is "Einmal eine große Dame sein" by Gerhard Lamprecht from 1934. The actress is Käthe von Nagy, born in Austria-Hungary as Ekaterina Nagy von Cziser, also called Katé Nagy or Katé de Nagy. The car is, as already mentioned, a Mercedes-Benz SS from 1933. In the movie Käthe von Nagy plays a car saleswoman pretending to be a countess. She meets a real barron (played by Wolf Albach-Retty, to the right of her in front of the car), whom she thinks to be a mechanic. Finally they marry.