I would make sure that the owner of the horse knew what sort of things you were planning to do with the pony, so you will have no come back if anything does go wrong with what he is being used for.
Shouldnt be a problem in a pony looked after ok. Plenty of top competition horses have real bad conformation and looked after just perhaps might have a shorter working life. The continentals actually quite like sickle hocks at least the are heading in the right direction!