I'd like to stick with a newer model (within reason) but they seem hard to find.
Well, that's the big problem. As the EPA and the NHTSA added more rules and more enforcement in the 1980s, the number of imports dropped substantially. Doesn't matter weather it was a Unimog or some racy Audi rally car, it became close to impossible to import something new on your own. For a few models--most notably for this group the G-series Mercedes trucks by Europa--the EPA and DOT licensed "authorized converters" that were allowed to try to bring the vehicle in line with US specifications. If they did it within, IIRC, a year, the vehicle cleared customs; if not, it got deported.
However, though a wide variety of circumstances, many Mogs did make it into the country. Some were here as "agricultural" or other non-road use, some flew under the radar due to some confusion with the accepted/prohibited listings, and so on. Things are much tighter now, with it being virtually impossible for an amateur to get a under-25 truck in.
But regardless of how they got here, once a Mog is acceptably in the country as regards the federal government, the issue then becomes whether your state will license and title it. That's not a challenge in some states, but in others, there may be additional issues due to emissions, safety requirements and so on. The wisdom is that if a truck has been titled and licensed in one state, it's much more likely to get licensed in another state, but even that is not an absolute certainty. The first titling of an imported Mog in the US is pretty hair-raising, though; in most states, the first thing they'll want to be shown is the Customs form approving the importation.
BTW, in Mog terms, the SBU series--1250, 1300, 1450, 1550, 1750, 2450--is considered "new," even though some are close to 30 years old. Be aware that there are very few Mogs in the US that are post early-90s.
