Welcome to the forums. I can give no insight on your first question but will try and help with the second. The value of a vintage Airstream is really what you are wiling to pay. After chatting with the PO of my motor home for sometime on the phone I told him we would but it and send out a check immediately. I hung up and he called me right back and with some embarrassment said we did not talk price. I told him what he was asking was fine with me and I had no regrets.
You did not mention the size of the Ambassador, right now it appears the smaller the higher the price. Larger trailers in the 70's did have frame separation problems which should be checked out. The Ambassador was the top of the line that year and should have nice equipment. How original is it? Does it still have the 8-track player? Do all th systems , like water, electric, propane work? Leaks, around windows and door? Have you bounced on the floor to see if it is solid? Does it sit level front to back, side to side?
You can find similar trailers in our classified, Fred's Page, and other Airstream for sale pages on line. Do a google search for 1979 Ambassador. Lastly if you really like it and this is the trailer you want the price is great.
Good luck to you, I love the 70's Airstreams -our first was a '75. Let us know what you decide.
Forest River Forester 2501TS