You sure are scrounging the Chihuahua desert looking for old aluminum! My general thought is that if it is a short trailer (22' and under) that needs a total restoration and the shell is in good condition, I wouldn't want to spend more than $3,000 on a very original unmolested trailer. This trailer has a bunch of strange modifications, missing windows, a dented panel on the from end cap. I'd give $1,000 for it, maybe $1,500.
Complete windows are not easy to source. You have to find them at salvage shops like rvrevive, Colaw's etc. or on ebay, forums classified.
Work is never done, so take time to play!