I would ask about the floor - if it was stripped down and replaced / insulated. I am not as concerned about the walls and ceiling as some- I've lived in my bus for two years, and the only regret I have was not better insulating the floor. As a weekend camper or just part time, I wouldn't even worry about the floor insulation - but I would hope that the original floor was replaced (if there was plywood under the rubber, that gets really wet / soggy / moldy).
The factory insulation was in really good shape for my 18 year old bus. My roof paint (I used BusKote from Hi-Tech / tropical is very good, too) greatly helped with keeping out the heat. The only real issue I had was the floor and the windows - the single pain windows don't do a lot to keep the cold or heat out or in...
For me, the interior is relatively easy to fix / replace - the engine is most important - so has it been sitting? Last oil change / fuel filter change? (I change my oil every 5,000-6,000 miles and the fuel filter every 10,000). I would also ask about the coolant - and if that was every drained, rinsed, and refilled.
In my opinion, with a strong engine and a solid body, you are in decent shape to make anything work.
- Roger (Dog.Rescuer)