Ringbone will prevent him from doing much other then very very light work and standing in the field.
The Cob I rode last year had ring bone and it was a lottery as to whether he was sound or not when we rocked up to a show! He could go onto the lorry sound and come off lame. He is now only just sound enough to sit in the field and sun himself.
So NO he would not be suitable as he cannot do dressage.
I'd say big "No". They are honest upfront saying he will not be able to do any heavy work or shows. Frankly he may not be good for anything but be a pasture pet, and I thought you were looking for one to show (if I remember correctly).