So here is a 'comp' if you will:
I think this rig ended up selling for somewhere in the $20-22k range, which was a great deal for someone looking for a low mile AWD Express with a poptop. The interior was a blank slate if I recall, but it had a window installed and came with a handful of decent parts. When we were in the market for an Express, I had seriously considered this but we wanted the CCV top bad enough to pass on this rig.
On the rig you are looking at, I'd be curious to know what other electrical systems are present. If it has a fridge/freezer, does it have an auxiliary battery? How is it connected to the engine's charging system? Electrical can add a lot of cost / complexity to a build. How many electrical amenities do you desire? How many days would you like to be able to run your fridge without having to start the engine or plug in (if you have a charging system to plug in to)?
Does the rear seat fold down into a bed? If not, is that a deal breaker? Sometimes people forget to put a value on a simple seat - especially one that converts to a bed. Can you go get one from a junkyard? Probably, but you'll have to look high and low to find one that is decent, and even then you may need to reupholster. A new one will be in the neighborhood of $1000. Want seat belts? That will be extra too.
The stove is certainly not a high end unit and the van obviously doesn't have on-board propane if you use the 1 lb green canisters to fuel it. Is that a big deal? Probably not if it meets your needs. Having a built in stove is pretty sweet and the coleman canisters used to last a long time with my old car camping stove so I don't see that as a big deal. The sink and water system would have little value to me; in it's current rendition I could take it or leave it.
Shooting from the hip, I see a $20k van + $8k poptop + $4-6k interior (if you were paying someone else to do it). I would guess that someone with a little skill and time would end up spending $1000 in materials on the general interior build + $500 fridge + $1000 rear seat. Add in $400-500 to have the rear window installed. It all adds up fast, but doesn't mean that you get 100% back what you've invested, which it appears that the seller is trying to do.
As mentioned above, see what options the van has. In our search it was hard to find a cargo van with cruise, PW, PL, etc. We also wanted the G80 limited slip rear differential which narrowed our search further.