Hi Bob, Here's some info that might help a small bit: Price vs. Condition - Airstream Values
Even though you say the trailer is in all original working condition, you should plan on spending at least a few hundred dollars on it somewhere. There are things that wear out no matter how good it is taken care of.
Check the curved side windows and screens, make sure the windows are glass and not plastic. Glass replacement windows run around $200.00 a piece not including shipping, and not including your labor. If the water pump is original, you would probably want to update it. The toilet valve can sometimes not be repaired if leaking, and a new toilet runs around $200.00. just a few examples. Check the exterior lights and lenses to make sure they are in good working condition.
Study as much as you can, before making your final offer. Do the brakes work, does it have a spare wheel and tire? Check the wall lights in the living room area and the bedroom. Does it have a working roof Air cond.? The article I mentioned says $3800,00 to $6500.00, I paid $5500.00 for mine and by the time I took it out camping for the second time had spent at least another 700.00.
The advantage you have, (which is recommended by me after my experience) is that the trailer is where you can really check it out thoroughly. I found mine on the internet and after many E-mails and pictures made a deal to pick it up out of state. We really like ours, but I was told it was all original, and later found out that the side windows had been replaced with plastic. I was upset at this discovery. The most unique thing about a 66-68 is the side windows.
Check for floor rot under the water heater and toilet, and under the carpet by the front door. Does the original door lock have keys? There are many things to look at that can make a difference in the price.. Hope any of this info helps.
There is nothing that can't be fixed (if you are a handyman, and can make the time to do it). Good luck, let us know what you end up doing.