Welcome to the club!
Look to the truck's weight ratings, both for GVW and the axle loads. Consider the tires, as they are also marked with the max each can carry. A little math will give you the truck's ability. Harder will be the trailer weights, as vintage units may have been modified, so the weight would really need to be from a scale.
What makes them hold their value? Used Airstreams sell very quickly at dealers and by private parties. Mostly, it is because they are built from durable materials. Airstream was founded in the 1930s, and at the time there were hundreds of other travel trailer manufacturers. Airstream is the only one of those still in business. Over 80% of all Airstreams ever built are in use today, from what I have heard in the factory tours.
Look up the Airstream Owners Club, at www.wbcci.org
and find the contact for the unit near you. These Airstream folks are friendly, and can help with information on the various years models and floor plans. The good, bad, and ugly, and how to make good better, bad good, and ugly beautiful is what the club can tell you, as well as show you.
The Forums do have a classified section, and, there are folks who are willing to inspect a trailer you are interested in, but not close to.
Good luck in your search, and let us know when you find your Airstream.
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