brunsonjc,
Seems to me, you're really keen on this. Understandable. Been there, done that.
I really wish I could answer your hypothetical question in earnest, but I can't.
You and your wife are both pianists, so I guess you know your way around a piano at least as a player and have certain expectations of what it should provide you with in terms of its performance, not necessarily yours.
Both of you should not be shy and think of yourself as a pain by sitting down and really playing this thing for a good while. Many buyers get a little stage fright in front of a seller, tinkle the ivories a bit and make a decision. Don't. Really play it, check for buzzes, knocks, squeaks,sloppy feeling or uneven keys, play in the pedals, anything that doesn't feel or sound right. Get real close, maybe even hover over it or under it while you wife plays and listen for the same. Check for cracks in the soundboard, the bridges, rust on the strings, the pins, the coils.
Again, that is as a player. The best thing to do is to pay a technician an average hours wages to check it out as a technician would. Its money well spent and even peace of mind if its lost and he/she tells you its not worth it or what would be worth investing in if you do purchase it.
The asking price seems fair if its in excellent condition. I checked around for that model for sale and although most were a bit newer, the asking prices were higher.
Asking is not getting. A G is a good piano, but Yamaha has made improvements over the years and again, its over 40 years old. Wood, felt, steel and copper wound strings, especially on an Asian piano of that era, do deteriorate, and at that age, they have for sure.
Wish I could be of more help and tell you what you want to hear, but I'd do you an injustice if I flung sunshine your way.
I'm a novice tech with only a few years under my belt. I'm hoping one of the more experienced techs on this board will either back me up or be more elaborate in their opinion.