My plan is a little on the overboard side, but not as bad as some. I'm planing on some Noico sheets .... to quiet the lower sound frequencies, covered with Thinsulate SM600L. ....and put Low E SSR on the back of these panels, to take advantage of the air space.... All contact points for the OEM panels against the van structure will have a thermal break.
If I were doing it from scratch, I'd get thinsulate 600 for the walls and 400 for above the ceiling (I have less than an inch gap). Thermal and sound protection, AND fire resistant, and also made for automotive use. No worries about if you used the "right" stuff, because it's the gold standard.
The conversion shop I've decided to use sold me, in part, because of the following line item on the estimate:
Three layer insulation package. Includes Fat Mat sound suppression throughout, 3M Thinsulate insulation and Low E insulation. Applied to all open sheet metal surfaces, including floor, behind door cards, above cab headliner and cargo area.
Note how similar to GapRunr: 1-sound deadening, 2-thinsulate insulation, 3-Low E where appropriate. In the follow up call, he also discussed the thermal breaks. Everything matched from what I've read (and experienced converting my own trailer). I think this is a top notch approach partly because, if you are running a conversion company, there's not a direct reason to go high end on both materials and extra labor that the customer pays for but can't see or really experience. So you'd only do this if you just want to make a great product. Another company I visited in person and almost selected I learned later (from these boards) uses fiberglass insulation.
I'll also note for the OP that windows are going to make a huge difference. If you have a wrap around view in a passenger van, I guess you'd still want to insulate what you could. But keep expectations low. Sun or snow, even covered windows are going to put a hurt on the effort.