I have finally found a manufacturer of composite camper boxes - the company is called BoX manufactur in Germany. The website at least seems to indicate they know what they are doing ('assume' makes an ass out of u and me!?) They do standard boxes, pull outs, lift up's etc. It will be interesting to see if its going to be realistic to commission a new build both in terms of cost and to see if the lead times will fit. I think I have a pretty good idea of what I'm looking for (systems wise at least - the 'Boss' will want to have lots of input about the internals and especially the bed / galley / kitchen). I want to travel in a much comfort as is reasonably achievable 'in the Bush' but the costs of good second hand units look a bit 'adventurous'. Maybe there's a business in building high spec boxes to shorten customer lead times........at this stage my lack of knowledge is woeful but at least I know that I don't even know what I don't know.
We could buy a nearly new Unimog 5023 from Mercedes Trucks and commission the camper box for it separately and marry them together. I guess we could even buy the composite body and have it fitted out in UK (or elsewhere) by one of the premium yacht outfitters as they understand lithium, water, gas, entertainment systems etc. Lots of options so lets see what my request for further and better information brings. Ultimately I want a robust high quality, low / easy maintenance product.