As a veterinarian, I have done a lot of pre-purchase soundness evaluations on horses for prospective buyers. I have come to believe there are different parameters to evaluate a horse's functional soundness depending on the situation.
I like to have a discussion with prospective buyers to come to an understanding of what they expect or need in a horse and his soundness. If it's a beginner or youth, it seems to me wise for them to get an older, experienced horse to start with. This situation often presents a horse that may have some changes that accrue with age and use. I feel it's my job to evaluate these changes and discuss their significance with a prospective buyer.
Examples might be a thickened extensor tendon sheath, or some subtle changes on radiographs that aren't threatening and may be considered normal for that horse's history. The way the horse presents at the time, the recent history of the horse's functioning and suitability for the rider are all to be considered in making a decision on whether or not to go ahead with the purchase.
I think that one should be less accepting of lesions present in a young horse or prospect. The time and cost to be invested in getting this horse to be competitive should be seriously considered relative to the risk of these conditions evolving to compromise the horse's serviceability. This philosophy also spills over into what procedures I would go ahead with to get a horse sound. To me, if you have a horse that's made and you're happy with, if joint surgery or a neurectomy is indicated to keep him serviceable, you go ahead with it. I personally wouldn't invest that time or cost, both financially and emotionally, in a young prospect. Perhaps a less athletic endeavor is indicated for that horse.
I guess my point to make is that soundness evaluation is not always a black-and-white situation. My observation is that people tend to want it to be black and white for various reasons. I have seen less experienced people pass up the opportunity to buy a solid horse that's 15 or 16 years old with some issues, to buy a 3-year-old that's free of any physical changes. That usually doesn't work out too well, but perhaps they learned something. The truth is, one has to accept that whenever you buy a horse, you're taking a risk. That's life.