Good luck on finding that trailer that's just right for you and getting into RVing. To most it is a fun and enjoyable adventure.
There is a PDI list sticky in the Keystone Questions sections (first item). You could print it and look it over. It's pretty comprehensive and will help you look for things you may not know about.
If buying used fits your budget then that's the way to go. I don't because it seems I always buy the other guys problems; others get great deals
Look the unit over closely. Don't know if you know the seller or not, and it's probably irrelevant anyway, but there could be hidden damage. RVs are notorious for water leaks and subsequent water damage. If the roof, window, baggage compartment and other seals have not been maintained that could be a real problem. We had a new member post just a few days ago of a used trailer they had purchased (19 if memory is working) and sounded/looked the the roof was shot due to a leak around the AC.
Good luck on the search. If you have any problems getting the PDI list post back and someone can help. Any other questions or issues just post away - someone can assist I'm sure.