I have an '85 Sovereign
25', so much will be the same.
The 80's Airstreams are an excellent choice. Prices are strictly by condition. If it is towable and has a decent exterior, at least $5,000; if it has a few extras and is really, really nice it could easily bring $15,000.
The factory plumbing was polybutylene, not pvc. It will be a medium grey, if original. It will also blow apart at the connectors one at a time. The tubing itself holds up great.
The converter (Univolt), if original, is old enough to be suspect. As is the refrigerator.
The tambour is of an age where it starts to come apart. But it may well last many more years. It depends entirely on storage conditions.
The abs end caps (above the sofa in front and above the beds in back) tend to crack along stress lines. There are no replacements. I patched mine and painted.
Check where the galley sink drain goes through the floor and into the grey tank. The hose clamp often is loose and allowing water to leak unseeen under the counter, rotting the floor.
The wheel bearings definitely need packing after five or more years. Five plus year old tires should not be trusted, even for short haul.
The curtains, if original, are likely friable. As is the upholstery.
And always, always, always check the axles!!! There are three of them at what - $800, $900 each?
These are definitely worth putting money into to make it what you want it to be. They are functionally equivalent to a new one, just not quite as wide. Take a look at my photos to see some of the changes I made in our Sovereign