Fit, first and foremost. If it hurts to ride you will not ride it. A lot can be done with seat post, bars, and stem, but a lot cannot. As @
50PlusCycling mentioned, too big is too bad. No way to fix too big.
Condition, next. If the bike looks like it has been used hard, then you cannot expect that it has also been maintained well Appearance is not so big a deal---bikes can be repainted---but scratches and scrapes, cracks if visible .... dents and dings .... Also, some stuff, like the bottom bracket .... shake everything, spin everything, because if the BB is not right---maybe the BB can be replaced, but maybe the threads are toast .... headset bearings can be replaced, but also, one could buy a bike which had a good headset .... if the head tube is ovalized .....
I assume from the outset that something about the bike attracted you ... whether it is a type of bike you don't own, or a brand or model yo have always wanted, or the price seemed phenomenal for the specific bike .... no need to consider that. If you didn't want it based on name, age, and/or price why would yo even consider the rest.
As for componentry ... I like Shimano shifters but have used others. No big deal. Com-ponents can be replaced (in fact, I might be buying the bike for its components---switch them out with stuff from an older bike in the stable, sell the new-to-me frame with the old parts, put the parts on the old frame ....
So really, fit and condition.