I used to base a PPE on the purchase price of the horse and what I intended to use them for.
While I was still horse shopping for my 2nd competition horse, a good friend approached me about selling me her TB gelding. I knew this horse VERY well, and often fox hunted with him and his owner. I was eager to purchase him and since we are such great friends, she offered me a fantastic price!
I was not going to get a PPE on him because I felt I knew him, but she insisted and offered to split the cost 50/50.
The day we met at the vet school, it was discovered he had a melanoma growing behind his eye. Its location rendered it inoperable.
So that PPE saved me a lot of heartache. Now I think twice about them :)
I'd always get a PPE, but I wouldn't spring for x-rays in cases like yours where there aren't any potential red flags (like being started and ridden hard early, etc.)
I know a horse that was recently sold by a vet. This horse had been one of two horses the vet owned and rode in eventing. The buyers did a PPE anyway (smart choice!) and found that he was showing signs of early cataracts. They liked the horse so much that they bought him anyway (using the cataracts as a bargaining chip I'm sure.) I know this vet personally and don't think she knew about the cataracts and was trying to hide them or anything like that. I think it's one of those things- just because she's a vet it's not like she's always examining her horses' eyes with a flashlight! So, even with a positive recommendation from a vet on someone else's horse, I'd definitely still want at least a basic PPE.