You will eventually need to form a relationship with a piano technician (the person who will tune your piano every six months or so and will do other adjustments over the life of the piano). You should form that relationship now, and have the person check it for you, and perhaps try to negotiate further reductions based on works that needs to be done.
That said, I think $8,000 is entirely reasonable for a KG-6, which, by the way, according to my notes is 6'10" or so. The 7'4" of that series, if I have this right, is the KG-7D. If it is really 7'4", well, the better!
A couple of additional notes. These pianos can be rebuilt Beautifully, so even if eventually it deteriorates a lot because of heavy use (say, your son get into it and hundreds of thousands of hours of practice later you need to replace the hammers, or do some other major maintenance), you still will have an instrument that either you can have rebuilt, or you can sell to an enthusiast to rebuild. And, you should consider that Kawais of this vintage are very very very nicely made. This is a well-designed and well-constructed instrument; I can tell you that without having to look at the particular specimen.
Good luck! Go grab it before someone else does!