I seem to be a fairly lone-voice on this, but I'd save your money.
Slap some flashband or similar to the inside of the bigger panels to take out some of the resonance and deaden sound, Then just carpet you lining boards and fit.
If you aren't making any heat, there's not much to retain and your van will not warm so well on cooler mornings as the insulation will prevent warmth coming in.
Others will disagree.
I admit, when I started lining our van, I did put a single layer of thin bubblwrap / foil to the inside of my upper panels on one side, but figured that it wasn't going do much, so didn't do any more. At least it doesn't touch the outer skin. 7 years on and many nights in the van, no issues.
Hot water bottles and a decent duvet on colder nights. I have put an eberspacher heater in recently, but still wont be adding insulation. Put wind deflectors on your cab windows and sleep with them ajar, to dissuade condenstion.
I used the shiny bubble wrap option (used behind radiators, I think) plus four layers of expanded polystyrene (used to insulate laminate flooring) behind the panels and the van is lovely and warm in winter. Keeps reasonably cool in summer, too. I agree with bigcats comment about the wind deflectors and condensation though... Nothing worse than waiting 40 minutes while the windscreen de-mists.