The usual things; full service history, not rough owner (Quintin "Rough people, rough cars), no homemade mods, not an ED30 unless driven by a little old lady (way too many crashed and repaired), not an 05 (catch fire), check DSG is smooth, don't buy a manual (too much risk of an over-revved engine), 5 doors not 3 for easy resale, red colour if possible (safer and less likely to be crashed), no sweaty smell, and finally don't pay a penny more for any of the costly original extras such as xenons, leather seats etc., they are not worth a bean to a secondhander.
Consider a set of secondhand wheels to fit winter tyres. After a 4wd subaru you will hate the wheel spin and axle tramp when the temperature drops below 6C and your summer tyres just don't work.
Burn the baseball cap.