Tips for buying a used O/U shotgun

[bat119]

I'm looking at buying an O/U shotgun primarily for upland game price range $1000 -$2000. I'm swinging toward a 20 ga. however there are a lot more 12 ga. around. there is a lot of option choices manual or automatic ejectors, single or double triggers fixed chokes or removable. Anybody have any suggestions what do you prefer?

I've looked at few I know to check for bulges and dents, some of the ones I have looked at have light what looks like pellet marks just forward of the chambers is this a concern?