I'm not going to be so "doom and gloom" as to say that if you don't buy it you'll regret it for the rest of your life, but I'll never be able to get back the Mav that my grandfather gave me, which he bought new in '74, because I rolled it when I was young and stupid. So I think destiny is at work big time when a situation like yours happens. Whether you decide to buy it or not you'll still have those memories and ultimately it's your decision, but I don't think I'd be able to pass it up.
Any new details yet? That is a truly wonderful story. My dad halfway would like to find his 57 BelAir, but I think it's pretty much impossible. It sure was a nice car. Unfortunatly I never got to see it as he sold it a little before I was born. He got it as a teenager and fixed it all up. Hope it works out for you. -Corbin
Nice restored SS Impalas are going for 15-20k, so if you can be done for 10k, it's a no-brainer, especially with the history. Maybe the guy will cut you a deal, being the original owner and all.