This one is easy! (I think?)
You said sister, so show her a picture of the cars like at, http://www.dhc.net/~pmhack/mercedes/models.htm
Then ask her is she wants the auto or the manual, 8 out of 10 times the women will say Auto, that will probably elimate the 190e.
Usually newer is better and gas as well, females usually hate the diesel smell!
Flame suit ON!