spare parts out of the uk are probabily non existant, difficult in the uk
look on mobile.de for merc 4x4 vans , theres an interesting already converted one there (814 i think) for 12k euro
you fall into the standard trap, there isn't alot of 4x4 options between 3.5 and 7.5 tonnes
if you went roof tent plus sleeping inside there are a couple of iveco 4 tonne panel vans around at present or what i think is a lifting roof one on mobile.de
Iveco Turbo Daily 4x4 40-10
Ex Owner LR101 300Tdi Ambi 'Tiggurr'