On the prepurchase exams, there's no easy answer. If I had of gotten a prepurchase on Winnie, I wouldn't have bought her, her x-rays were horrible and of course, she turned up lame.
I got a prepurchase on Tuffy and his was iffy, he does have some problems, but I wanted him so badly I took the chance and never regretted it. If he would have had any major problems, I wouldn't have bought him.
I've only sold a few horses, 2 of them went back to previous owners (Juma's mama and a gelding I had), 1 was Juma and I had him priced so low he wasn't much of a risk and one was a palomino gelding that was basically given to me. I sold him to a friend of my brothers, very cheaply, for him to lead his son around on. This was about 5 years ago and they still have that little horse. No one asked for a prepurchase but I sold the horses so cheaply it wouldn't have made any sense to go to that expense.
I did have the vet check Sarah Lee out but just basics and immunizations. Like you, I already planned to buy her though unless he saw anything major that was wrong.