Unimog Club UK

Welcome aboard Wolf were here to help where we can. If your chassis is in pretty good condition with just a few spots of rust you could have several options, you can do what you said that is paint and prime this works but is lengthy and if not done 100% can look bad later with paint peeling and rust creeping under and around parts mounted to the chassis.
My preference is steam clean wire brush any lose rust treat with rust converter that turns the rust black then spray liberally with dinitrol which also contains a rust converter this is what Mercedes used to spray the chassis with its a brilliant product and looks good and protects for years I'm also a fan of what Mercedes used to recommend before the world went environmental they used to recommend once a year power wash and spray 10w oil all over your chassis I've worked on old Unimogs that where done this way and they are a joy to work on.
This is all assuming you want a working truck not a show truck.
Saying that it still looks good, and to do one to show condition one should take everything of the chassis and paint individually as it did when it left the factory.
Moose