Welcome to Bird Forum. I know a fair amount about Swift Audubons, but not much about astronomy. Still, I'll venture a few comments that might be of assistance.
The large body Audubons of the 1970's were fully coated, but not multicoated. They were very well made, but unfortunately prone to flares and reflections. The original HR/5 models issued by Swift-Pyser in the early 1980s had multicoatings, but not full multicoatings (see pgs. 14 and 16 of the report posted
HERE). So, the value, in part, depends upon which HR/5 is available to you. Nowadays I think a Fully Multicoated HR/5 in excellent condition would be worth what's being asked for it.
Now, with regard to astronomy tasks, consider that all objects in the field are at optical infinity. Since the Audubon design does not incorporate a field flattener, how much appears to be in focus will be determined by the instrument's inherent field curvature and your visual accommodation capability. If you are above 50, for the most part your accommodation will be very limited and the outer field will seem to be out of focus. Fortunately, unlike other binoculars, the Audubon has very little astigmatism, so this can be largely rectified by refocusing the instrument.
Personally, I would recommend the newer HR/5, particularly if it's fully multicoated.
Ed