Unlike cars where the new model years are introduced in September, new year mobiles are often introduced in March or April. Although in the years of the code changes, that could be different. If it was me, the one thing that I would look for is copper wire. If it has aluminum, I think I would pass on it. As you said, you can determine if it has 4" walls. It it has an outside receptacle, if you pull the cover on the box you might be able to see sheathing...or..possibly by pushing on the siding between studs you might tell if there is something solid behind the siding.
If it was me I would worry about the wiring as I mentioned, I would check for flooring soft spots in the bath, under kitchen sink and in front of all windows and doors. If it has a flathish roof, I would be checking the ceiling for signs of leaking. You might want the furnace checked by a service person. The chambers crack over time and people continue to use them.
Since i was in the business of mobile home parks, I have probably owned 75 of the vintage (and older) that you describe. These are the problems that I have encountered.