Largely what you'd expect. The van is an older model so has "old car" quirks that aren't too bad if your expectations are set right-- things like a window that wants to sneak down unless you open the door while you close it, some rattle while you're driving etc. Nothing overtly bad, just might matter to some.
We had no air conditioning for our stay because Tiago forgot to check whether the previous guests had drained the coolant. He was apologetic but that sort of thing should probably be on the checklist before handing off the car.
As for the interior, the remodeling done is impressive and includes features great for a few nights of sleep. For a sense of scale, I'm 6'3" and my ankles hung off the bed so it's good enough for most (I was surprised I fit it that well).
If you can, booking a campground in advance removes the worry of finding a place to park overnight.
Overall getting a car to explore O'ahu in and having the added bonus of it being your overnight housing is sweet. There were parts of the car we didn't need but were there--the heated water out of the shower head in the back, the camper stove--so it's pretty well kitted out.