Greetings and welcome from Colorado: And maybe welcome to the vintage Airstream hobby. I have a 75 Overlander 27' which is two model sizes shorter than the Sovereign
31'. The price you mentioned is a "family price" seems to me as I would guess the trailer is worth twice that as it sits in your photos.
That said, Airstreams in the damp northwest usually have more moisture damage than a southwest trailer. For some reason, 70s trailers are prone to more frame rust that other decades before or after the 70s. 70s trailer were built for light weight due to the high cost of fuel and early emissions controls on vehicles robbing power creating smaller vehicles. I find the cabinet materials inside a 70s trailer tend to delaminate and fall apart. The plastic bath parts and plastic end cap ceilings tend to crack. 70s trailers are more work to renovate. I did my 75 Overlander and then my friend and I renovated his 76 Sovereign
. The gory details can be found in the Knowledgebase near the bottom of the Forums page.
That said, these vintage 70s trailers can be renovated into very comfortable travel trailers, and it is worth the effort in my view. I would guess this Sovereign would cost about $15k in materials and 1000 hours in labor to complete. Think axles, brakes, tires, water tanks, belly pan, frame repairs, subfloor repairs, new plumbing, new appliances, cabinet rebuilds, floor coverings, soft goods, door lock and on and on. It's fun but a big project. If you don't think you would like laying on your back under a dirty old Airstream, maybe this isn't the way to go.
Here is a tour of my 75 Overlander. It tows nicely, and it is very comfortable for us.