This looks super incomplete from the pictures. You are just getting a few extra things you won’t need to buy later. It looks like a low top so you won’t be able to standup. I would get the mileage and value it against the current market price for its condition.
In my opinion not a lot of work was done here that could not be done yourself in a few days. The additional insulation is a waste unless you have somewhere you can use it.
I think unless the extras are being thrown in at a heavy discount, you’re not much further ahead than you would be getting it this far yourself. If you wanted to add any windows you’ll have to undo part of that work as well.
I would take into consideration the sellers competence level as well. Did they have a clue what they were doing or simply winging it? Don’t want a leaky roof vent.