I've had a handful with 70+ starts, and none that I recall had soundness issues from racing. I would be concerned about arthritis, but otherwise, these are your iron horses.
Had one that started 94 times, then did pony club, hunters, and evented at lower levels into her 20s. She was never lame, sick, or injured, and died of natural causes at 29. Granted, this is anecdotal evidence, but these horses absolutely know how to take care of themselves.
Pre-purchase exam would not be cost-effective relative to sale price...
It costs just as much to board and care for a free horse as a $100,000 horse. Horses bodies don't adjust their needs for how much a person paid for them. If you want a usable horse then make sure it's usable.