I have a 1977 Avion and am in the process of purchasing a 1966
Airstream. The Avions have nice interiors; I love the real wood. Airstreams are "cooler".
Whatever you do - look at the roof. At least ask for photos. I got taken on my '77 Avion. Everything looked great, but we neglected to check the roof. Dumb, I know; live & learn. When I got home I found tar poured all over it to cover up gaps and missing rivets. I've tried cleaning it up and have just given up on it. This morning I went to look at another '77 Avion. This time we climbed up on the roof and found it in the same condition. We just walked away feeling much wiser.
You are right about parts. My Avion needed a new roof vent, but they are that funky 18" vent that no one makes. I did manage to order a new vent cover from Cayo, but the rest of the vent (screen, fan, motor) can't be found anywhere. Curved windows in the front can't be found, nor the window cover that protects it when you drive. Airstream parts are easy to find. Especially here in Austin where everyone is gutting them to convert to food trailers.