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.