Hello TAE - welcome to the forums! I've moved your post to a standalone thread.
Has the '77 you're looking at received any regular usage? It's hard for them to vouch for the air conditioner unless they've actually had it at a campground 30A outlet and operating. A fridge replacement can be one of the most expensive single maintenance issues. An original furnace must
be replaced as a simple safety issue. HW heater probably isn't doing real well. Original copper pipes are subject to irreversible frost damage unless the unit was very well maintained.
And finally, like any 'Top 10' list, the most common and sometimes largest issue is, "They all leak." That's just impossible to avoid. It's hard to imagine how any inspection could have found two of the four floor rot problems I had. Hint: look very carefully around the battery compartment -- visualize the floor under the wardrobe in front of the Univolt.
Good luck in your quest.