From reading your post, you seem to have your head on straight as far as what to watch out for (floors especially). You have hit on the biggest issue in considering the longevity of an Airstream: leaks! Leaks will make the trailer much more costly to bring back to as-new condition.
So, of course, the '80s AS units are just old enough to have all those weak floors issues, but the only other thing I can think of that, as a group, they are fairly lightweight.
Here's a few:
'68 Ambassador 28' twin: 4,395 lb
'75 Ambassador 29' rear bath double: 4,855 lbs
'80 International 28' twin: 4,143 lbs
'80 Excella II 28' twin 4,443 lbs
29' centre bath 5,300 lbs
'88 29' Excella 5,600 lbs
'93 Excella 29' 5,600 lbs
'96 Excella 28' 6,720 lbs
So you can see, a lot of the '80s TTs were comparatively light weight, which can makes for less expense in the Tow Vehicle.
People will say that in the early '80s the Beatrice Corp had taken over AS and they ordered that less steel be used in the frame, but overall, I am not fully convinced that had any large impact on longevity. There are tons of '80s still around.
Keep looking, there are lots around, and remember that there really isn't much (if any) difference in pulling a 31-footer than a 28-footer, but I seem to see way more 31' units than the 28'/29' ones.