Hi Karl,
You can probably guess I'm totally in favor of your upgrading to a U-1300!
For those few readers who are saying "what's a U-1300 Unimog", here's a really well written summary of the Unimog SBU group of vehicles including the U-1300.
http://www.unimogcentre.com/unimog435.html
Trading might be a challenge. The 404 being a common introductory-level Unimog due to lower cost (usually less than 1/4 of the cost of a typical U1300), most 404 owners want to upgrade eventually. I don't know of many reasons to downgrade except size. The 404 is the ideal trail machine where an SBU would never fit.
Expedition Imports
http://www.expedition-imports.com/ probably has the largest SBU inventory in the states. Scott just brought in several U1300's. Of course there are many other sellers here too.
If you are contemplating using your U1300 on the road mostly, then you might consider fast axle ratios and even the Claas overdrive. The OM366 turbo'd engine is a real jewel but the OM-352 is a solid engine, too, and actually revs higher (faster road speed) than the '366. If you can use your SBU for farm work, then getting an agri-equipped SBU with PTO's, multi-hydraulic circuits, and hitches front and rear really creates an awesome work machine. Agricultural Unimogs might be more plentiful in your neighborhood.
Bob