Hi Bob,
Welcome to the Forum

Regarding campervan conversion, to be able to register as a camper there is a number of requirements, we followed this and our truck got through with flying colours:
http://www.campervanlife.com/building/legalI realise that it says the conversion has to look like a camper, however, our truck looks to us like, well, a TRUCK. I think what they mean is that it needs to have windows and a door, and perhaps take a picture with some steps into the back.
Limited use is frankly a real pain, basically it means you can travel a certain distance (If I remember right a mile or two) from the work site, I believe it's intended mainly for the likes of farmers who have an old discovery that they use to carry a bale of hay to a field. this obviously causes issues for the likes of yourself who intend to use a Unimog for long journeys.
I would imagine the reason it is registered as limited use,is because someone has attempted to register it as agricultural, and the lack of any linkage or other agricultural equipment means the dvla have decided to register it for limited use instead.
Cheers, Brian