Hi bob, and welcome.
From you brief description above of what you want to use your unimog for and your concerns of running legal, it sounds like you set up would not could under agricultural use, I`m guessing muck shifting for hire or reward, delivering plant, delivering bulk bulks. while this is all possible and legal with a mog it might need to be registered as a commercial, bringing o licence, fuel, driving licence, so on, into the mix.
Going back to the choice of unimog I would go with the others and say you would want a heavy chassis square cab for lots of towing, there are some versions of small chassis but with big power but had to come by.
Towing the weight you talk should be OK as for a correctly set up agricultural machine can gross at 24490kgs (of the top of my head).
Sorry If I dampened your bonfire down slightly, and apologise if I have read your circumstances wrong and you could come under agricultural use.