How old? I wouldn't go back more than maybe 7 or 8 years, if that. Around 2013-2014 they had oil burning problems due to defective piston rings. Early Subaru CVTs were weak and prone to failure, and conventional automatic transmissions on earlier models were nothing to write home about. (Manual transmission is far preferable!) If you go back to timing belt engines (1996-2012) you have blowing head gaskets to be concerned about, plus expensive timing belt changes. You wouldn't want the "H6" flat six engine, it's a gas hog and very expensive to repair.

Basically, the newer the better.

I agree with @chucktobias about the H6. Other than that, the quickest way to answer your question is to look at carcomplaints.com.