Can't really give advice on heaters, but I'm going to have propane and wood.
As to insulation; I don't see how buying any manufactured van off the show room floor or used from average camper type would give you a good start for an extreme weather rig. Not unless you gutted it and started over.
So, what's the budget?
Blown in insulation will be the best way to go, IMO, but I didn't spring for that due to cost and I'd still end up having to modify windows and doors.
I used Harbor Freight floor mats, they fit together like a puzzle and when you open the package they stink....but let them air out for a week and they lose the odor. They are water proof, they don't soak up at all as I did a submerged test for 5 days, I couldn't get any water out of them.
Liberally use 3M90 to glue these mats to the interior surfaces to the metal. Then cover with blue board insulation, then reflex and tape it up using the aluminum tape. I then used corrugated plastic sheets 10mm thick. These sheets do have some R value but they provide an air space over the reflex and under the wall material, Not tested yet but I suspect this will help any condensation issue with the walls. BTW, I used bat insulation in pieces in all the interior framing to cut down on noise and air flow.
The floor got a paint job for future rust, then blue board, then mats, then reflex, then plywood then flooring, mine got carpet.
The ceiling got two layers of reflex without plywood, plastic sheets will go over the reflex then ceiling material.
All ceiling and wall fixtures have tape on the wiring penetration and fixtures are screwed in over the ceiling material, so the only conduction from the van body is by way of the small screws in the fixture.
I'm not using any wood battens or stringers inside, if I did use them they would go over the reflex, not on the metal body.
The front of any vehicle is an issue to insulated, my solution is a partition behind the front seats, if I were to have swivel seats it would be from the floor to ceiling in front of the dash.
Again, good ole Harbor Freight moving blankets, I'm using two layered, sandwiched in reflex then I have a PVC coated canvas used for Army tents, you don't sew this stuff you glue it. I'll allow for lapping this against the ceiling, floor and walls held by velcro. This partition only goes for extreme heat or cold and rolls up to be stored in a roll on the ceiling passenger side wall, out of the way.
Windows get custom covers that cover the trim a couple inches onto the walls or doors. My layers will be two layers of reflex, then the mat material, then the plastic sheet, all glued, the walls material, fake leather stuff in my case.
Next is to address door jams, another weather stripping besides the rubber gasket.
I'm considering another partition type panel to cover my double doors over lapping the threshold/floor, jams and header, they can open with the door if I need to bail out.
If I install a vent fan a small cover will be made for that as well, I may not since I'm planning on a marine side vent that can close which vents my fridge and kitchen appliances, it will have forced air from a computer fan.
I'll say my cold weather experience was from being stationed at Ft. Drum, we'd go out for a week in Jan/Feb and dig in snow caves, it got to 52 below with the wind chill so, there ya go, just don't kiss your door handles.
