Hi, and welcome to the forums!
I love your list of requirements: "reliable, towable, and value-laden."
The first two are achievable--I'm still debating whether any RV can qualify on the third point
Anyway, no harm in asking members to peruse an ad for a trailer. Having looked at the one you referenced, I would say that the entire list of features was cut and pasted from a sales brochure, and the question is what condition is the trailer in now that it is 40 years old?
It looks nice on the exterior, which is a good start. It looks "original" on the interior. Some people may prize this as it means the previous owner didn't molest it too badly. My concern is that if the previous owner never replaced the 40 year old carpeting or upholstery, then I would assume that it is sitting on its original torsion axles (the rubber in them does not last 40 years), has had no modern upgrades, and very likely has issues that are typical of 40 year old unrenovated trailers (floor rot, frame damage, non-functional appliances, etc.).
Click on the "portal" tab at the top of the page, and when you get to that page, you can download a buyer's inspection checklist. The checklist will help to to know what to look for, and what questions to ask. In general, I would say to ask what improvements/repairs have been made to the trailer. The problem with this trailer is that it is being sold by a dealer, so you are probably going to have to figure out what is new and what is old by your own inspection--they probably won't be able to tell you.
In general, if you want a trailer that is ready to camp in, then expect to pay $10k+ for it. The value in an old Airstream is mostly in the shell. Those are worth a few thousand dollars. The way you get up to the $10k+ price tag is with a lot of labor and replacement parts. If there hasn't been a lot of labor and replacement parts, then it probably isn't worth what is being asked for it.
Also, there are Airstreams with bunk-beds--this may help you to get the sleeping capacity you need.