1. Sovereigns are plentiful and should cost less
than a shorter Airstream.
2. Tail sag is bad, but can be fixed. You have to remove the interior shell at the rear (and all the fixtures that are in the way of doing that) in order to do a proper fix.
3. The shell dings (front dome, above bumper, belly pan) and missing rear bumper/locker will be very difficult to repair/replace.
If you want a first class vintage Airstream when you're done, this is not a good choice. $4800 would be fair for a Sovereign
with a near perfect shell and working plumbing, but is way too much for one in this shape.