Of all the options you have outlined, I think the Mercedes 4x4 tractor will be the best base vehicle.
They were reliable if unspectacular with that engine, Also, from the photographs it seems to have steel suspension and drum brakes, which is a big advantage on a vehicle which is not in regular use. and from the description it seems to have a conventional four wheel drive system; ie no hydraulic drives. Biggest disadvantage is the short wheelbase, but I think you would be able to get a body approaching 4.8 meters long on it without cutting and stretching the chassis.