First of all, Welcome to the site!
You could probably "google" for Airstream trailers to get your answers. Or try the forum on vintage trailers.
The biggest issue is the initial build on the unit. Was it sold as a "Joe Six-pack" low end unit? If it is used and five or more years old it is probably not worth looking at. It all depends on where the unit was taken for vacation. E.G. boondocking or strictly private RV parks.
Buying a high end used unit is often the better idea. There are a ton of used RV for sale web sites. If you look at enough of them you can determine the value of the unit that you are looking at.
is a good place to start.
NADA and KBB have some ratings on prices fro used RV's as well.
Good luck in your search.
p.s. Look at the classified ads here as well.