Well, I can say that if I were to go out looking to buy a horse for a child, I would not buy him for more than 200.00. I would have to spend 6 months to a year working with him on a daily basis to get him desensitized and safe.
I would try to work with him on driving. I think he would be a cute driving pony, and if that is the case he could be used by an adult.
Ponies can carry more proportionately than a light horse- he is stocky and thick, I don't see a problem with him carrying an adult rider.
Spent a whole hour today laying in a pasture, waiting for a sparkling vampire to show up. Alas, I woke up and looked over, only to find a mound of horse crap. Sigh.