Restoration prices can head skyward when you have to remake significant parts of the frame. This might be worth it for a '50s or '60s trailer of some value (frequently the shorter lengths). Many members find that 1990 Airstreams can be found in quite good condition, requiring some general upkeep while you are on the learning curve.
You can put a lot into a restoration and only have a modest value trailer in the end. Of course most of us love our Airstreams when we actually get to use them, not prepare them for the next owner. See VAC's Price vs Condition
and Photo Archives
Search on 'rear end separation' -- this tends to be longer trailers from the '80s when Beatrice Foods held the company. The skin would have some wrinkles aft of the wheel wells due to inadequate frame strength.
Look at thread after thread. Look in the 'Forums' area at top left to see how ASForums is organized. Use the search function on topic & values. Begin to cruise the ASForums classifieds
to see what turns up. VAC has classifieds
too. The Maw of the Monster (eBay) might only chew you up and spit you out! You'll have to be patient in the search -- any extra time you take in the search will be rewarded by much less restoration time!