What is your tow vehicle? Id look for something around 26-28. I have an older 28' model that will sleep 6, Im not sure if there is a certain brand to stay away from. Most common older ones are Salem (forest river). Ours is a 94 but is mint shape inside and not bad outside. Either way as long as the suspension is not rusty and the tires are good youll be ok.
Plug into a generator and verify the ac blows cold,fridge gets cold and the water pump works.