Hi treefirm u1650 full ag spec are getting long in the tooth now unfortunately the cabs are suffering with rust issues we have one in at the moment with a full cab rebuild it's needed a lot of work. You need to know where to look as a Unimog can look fine from the outside.
Mechanically the 1600 /1650 are a durable Unimog look out for the usual achilles heel the portal hubs ideally you need to drop the oil and inspect this after a short drive or jack each corner and check for play.
If I hear of anything I'll let you know on here.
Moose