Will follow your build with interest. I'm just starting the same process here in NZ too, albeit on something less adventurous than a unimog. Will put a styromax/vanglass style box on an Iveco Daily chassis cab.
How goes the search for composite panels? Im talking with composites group in Christchurch but im still tossing the idea of importing from Aus given their pricing and quality.
4.2m is pretty high, im from the Otago area and a lot of rail bridges are around 3.8- 3.2 if im remembering right. Theres always easy enough detours so I suppose its a convenience factor.
I think I made some fundamental errors in the model of the truck , so I think more likely to be at 3.8ish.....
I haven't yet started the search for panels. I have seen one camper made from a roofing product with steel/foam/steel in Auckland, and there is Clark Global (https://www.clark-global.com/contact) who can supply panels and other parts.
I have some experience with Iveco...... that lap of the South Island was not a success, but with some further work I just did a lap in Feb, a very rushed 6 day lap from Picton through Queenstown and back for a MTB trip with a mate with no issues.