Sounds like the guy might be trying to break even or make a little extra for his time of putting the stuff in it. I would say you can find a newer bus and put in the stuff you want for less than 4k, unless you would rather just jump into one that already has what you want.
I paid a little more than I should have at 3000 but I wanted to keep it under 30 feet and this one is only a 25 footer and has a front wheelchair lift door which is nice if you need a lift or if you don't need a lift. Having it makes it a lot easier to load stuff in to the bus and if you want to put that newer house couch in the bus after you have the back walls in you can do it without removing a windshield and lift it to get it in. In my case I need the lift for my friend to make it easier for her to get in/out and we can also take her power chair in the bus and then I can leave the truck at home. (I use a 8 foot trifold ramp to load it into the back of the truck) I can't forget to mention with the price tag I did get a very good running 7.3 motor that didn't burn any oil in the 600 mile drive home with truck in tow. And the only rust on the thing was some surface rust on the frame and calipers. And it also had a new service door because I think there was some kind of collision but they had repaired it very well and you can't tell it was ever hit there unless you look underneath it in that area. My only down side is 5.57 in the rear and that makes a 600 mile drive visually boring but it is easier to look at a map while driving and I didn't have to worry about going over the posted speed limits. It will get re geared at some point I just don't know when.