From your description I think both trailers are in similar condition. any trailer of this are will have "bad" axles. Even the first one may look good in height but will most likely not have any travel left in the suspension. I would budget on axles in either case. Right now complete units are on sale for $595.00 each plus freight. Labor will be around 4 to 5 hrs maybe a couple of hours less and that cost depends on the shop rates. Also budget on new shocks and mounting hardware, another couple of hundred bucks. My WAG is close to $2K installed for both axles.
Inland RV Center Inc.
To answer your first question about rear end sag. A little sag is like a little pregnant either you are or aren't pregnant and either it has seperation or it doesn't. Cost to repair is very subjective depending on the shops rate per hour. The A/S in the pic is a rear bath unit which requires quite a bit of disassembly to repair, however it is more complete as you describe.
Both units sound like they need major floor replacement. If you gut the interiors and pay a restoration shop to fix the floors and the frame (both will need frame repairs) then budget on at least a few hundred hours to do that, which can easily can reach $10K.
Any Airstream dealer carries that window and at price tag over $350 but you may be able to find it at a salvage yard for half that amount.
RV Salvage Parts Airstream Trailer Interiors Restorations Collision Repairs