Kevin seems to have given you all the history to get you started.
Just to let you know, I have restored a 16'
1963 Bambi, a 24' Argosy, a 22' Argosy and I am now working on restoring a 20', (6 Metre), Minuet.
I have also worked on several other Airstreams and a couple of Argosys. My entire family owns and uses these trailers. As a matter of fact, my brother-in-law now owns my late father's 1973 31' Airstream. He bought it brand new in Nov. of 1972. With the proper maintenance these trailers can remain trouble-free for years. I repeat, WITH THE PROPER MAINTENANCE.
Keep in mind that even though the Argosys are less expensive that you will still have to invest time, money and sweat into any trailer you purchase. An Argosy may take the same refrigerator as an Airstream. That means that if you have to repair or replace it, it will cost you same amount of money and work to install or repair it.
I am willing to bet that as you see the older Airstreams get sucked up by collectors the price of the Argosys will increase. It is the simple fact of supply vs. demand; fewer vintage Airstreams means bigger price tags; bigger prices tags means that the Argosys become more appealing; Argosys become more appealing and there will be fewer Argosys; price goes up. Think about it; you are contemplating an Argosy for the same reason.
I got my Minuet because it has a really cool aluminum floor! I show it off to everyone! Some are amazed, others are bewildered as to why I am showing them the floor of my trailer!