If they are set and cold, and the horse moves the way you want to see him move, they should not be limiting for what you want to do with the horse. Xrays will show the position of the calcification in or around the joint, if you want to spend the money for that. It may be clear of the joint, or partial fusion, or full fusion, and the horse CAN still be sound. I rode a horse as a kid who had a fully fused ankle with an osselet, and he had been jumping in the 4’6" division for years. I rode him in the junior jumper division. From me, he went on to be a school horse for nearly another decade. Any horse you buy does not come with a soundness guarantee. Eventually, you just pay your money, and take your choice. If this one speaks to you, and is priced right, he’s as good a bet as any.