Hello wannaroam -- and welcome to the Forums! I wanted to post back to your thread for no other reason than my father grew up in Clarion. I haven't been back to the area for about 20 years though I have many fond memories of roaming with cousins while growing up.
One of the basics is this Price vs. Condition
webpage. At the top also note the link to photo archives!
Unless actively maintained you could bet on multiple areas for restoration and repair. You would be well rewarded in the end. It could take a significant investment of time and you will ask yourself, "Why am I doing this?" Therefore I would recommend starting on an Airstream size and layout that truly fits your desires. If this is it, I say go for it! Sweat equity and dollars invested in restoration could double the purchase price (or more).
You'll see deterioration of window gaskets on any unit this old. Has this Sovereign
been stored outside or inside? If stored outside you might want to open all storage, gaucho (couch), and underbed areas to see as much of the plywood perimeter as possible -- chances for leakage and floor damage can be high on a unit this age. Just imagine any other brand at half that age -- it'd be ready for the junkyard.
Vintage Airstream maintenance can turn off some RV or Airstream dealers. Talking with other RV owners in my area, I have found a few smaller local RV repair shops that realize the basic systems are all about the same. An owner can do a lot if they are inclined that way. You will need to have new OPD-compliant LP tanks and -- about $70 a tank
at Camping World. CW is a fair starting place to check on other prices too. The most expensive replacement item would be if the refrigerator doesn't work. Air conditioners this age can be problematic -- don't use a 15 amp adapter on the power cord to try it out -- take it to a campground and use a 30A plug-in or you'll risk the compressor. Original Suburban furnaces
of this era were a problem -- getting new is relatively inexpensive and good peace of mind! (some members very good in appliance repair have re-used the old furnace after proper modification). Hot water heaters? My better half started looking at curtains & upholstery -- best put that off until you've worked through a lot of the more basic assessments. Just stand back and look at the shell -- if you see a thing of beauty I hope it will carry you through all adventures.
The search function here is a good starting point. This is a self-help community and I'd encourage you to keep asking questions. Every viewer here has had their knowledge advanced. Let us know if you jumped at the Sovereign
opportunity -- and come back often. Happy trails!