I contemplated a Luton van as the sides present less problems, Easy to insulate, but the roofs are likely to be quite flimsy to keep weight down al the ones we looked at had see-through GRP sheets so would need to be reinforced to take solar panels etc, but not difficult to do, I have seen since some really nice jobs done on them some have a drop-down rear providing a deck to sit out on up out of the mud with patio doors at the back, wood stoves are a nice thing too so keeping warm should be easy enough but no forest parking, as David say doors aet are easy to find, plenty of crash damaged caravans around at the end of the summer.
A lot depends on your skills of course, th hardest is the joinery for most, the rest can be farmed out as you need them doing.
If you go for a panel van look on Ebay as there are a lot of kits for a simple build, you just fit it in, but I don't think they do bespoke unless at a premium, there are sometimes unfinished projects on ebay too, worth a look as you get a lot for your money.
I Plumped for a self build L4 H2 Citroen relay.
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