I have maybe 700 hours and over $8k renovating my 66 Trade Wind 24' twin. It sleeps 3 comfortably. The frame and shell are in pretty good shape. I like the size and the tandem axles. But the 66, 67, and 68s have odd ball Phillips frame less windows with Corning curved glass. Parts are hard to source.
The desirable Dura Torq axles started in about 1962
, so I would want those. Tandem axles make towing a bit more stable, so I looked for models that had dual axles. 1969 was the first year of the new body style. The 70s trailers saw "design changes" that lowered the manufacturing costs. They still have the desirable alclad aluminum skins, but the interior parts were cheaply made. The wing windows in the front are hard to come by. Many Airstream renovators have made beautiful trailers out of 1970 models but it probably takes more work and more costs to do so.
I think build quality was improving by the 80s except for the OSB subfloor materials and softer aluminum skins. Our 86 Limited has beautiful hickory cabinetry that has stood up well through the years. My 86 has minor hail dents on the end caps where my 66 does not, both trailers have been in the same weather for about the same number of years. Alcald aluminum seems stronger, and I'm sure more expensive.
Bottom line: A longer sixties trailer would be a great project. A longer 80s trailer would have the upgraded electrical wiring (not aluminum wire!) and might require less work. There is a good chance you can get your parts money back out of the trailer when you go to sell it, but you won't likely get paid for your labor hours.