In reply to Siward:
I had to make the same decision last year when I was doing mine:
Foam: seems cheaper, seems easier (just blast it everywhere) etc. However, I've heard it has a tendency to go soggy and obviously once it's applied it's a big mess behind the panels, so if you ever need to un-insulate the van for whatever reason, it'd be a nightmare.
Foil: more expensive, a lot more hassle. Generally much better at keeping water out so long as you seal everything in properly. Much neater, theoretically you can strip of the insulating panels (at least if you're smart about it).
I went for the foil in the end - I did the following:
Get flashband from Screwfix - tape over the bare panels as much as you can (within reason, it's expensive stuff) I focused on the wheel arches and on the thinner panelled parts. i then applied a layer of 3mm soundproof acoustic underlay everywhere (ceiling, walls, wheel arches, floor). I then sandwiched panels of Kingspan-type stuff between two layers of foil bubblewrap and sealed everything in with foil tape.
It's worked a charm so far, van is pretty warm at night (though, as has been mentioned, you lose a bit of heat through the cab).