g/f's tradein

my girlfriend is looking to trade in her 2001 Mustang GT (Auto.,17K) at the end of the summer and I sense she wants some kind of "luxury" car. Trade in on blue book was like 15K and who knows how much her father will spend. I'd say no more then 20K. What years or models of cars would you look at....i'm thinkin BMW, Mercedes, Audi,Lexus?.but not sure what years and models to expect for 15-20K
Any information or bright ideas would help me browse lol...