You don't know what you have in an Airstream until you pull the belly pan and inspect the frame. The floor can be sound(plywood) and the frame rusted away.
I can read the manufacture date on the plywood floor of my 1975, but I am building a new frame as the original one was eat up in rust.
The appliances are 37 years old. How long do you expect the A/C, Refrig, Furnace to last, These three alone will cost you 2500 to 3000.00, plus labor.
At the price the owner has set you would be better off buying a refurb. 9000.00 trailer with a history of documented rebuild,repair,replacement.
Have you checked the axles, if original or 15 years old there's 1500
.00-2000.00 plus install labor.
The worst thing you can do is fall in love with this trailer and over look the faults and necessary repairs.