I can see how steeplechasing has helped him in one aspect at least. When you watch him jump he is very quick to pick up his front legs, and he also kind of kicks out his back legs as well. Which is certainly a good thing in Jumpers and eventing, because you know he is both tight with his front end and rear end. A lot of horses let their rears hang over jumps. He actually seems to make an effort to really get over it.
As far as 15,000, I agree with what others have said. He isn't worth that. Has he done any showing? Have they taken him anywhere? Possibly if he was already a proven winner, then they could ask in the teens' for him. But right now, as he stands, no show experience I am presuming, I'd say anywhere between $5,000 to $7,000 if he is super quiet as well.
He is a little short backed, which is why he has short strides. Not a bad thing, but he'll never be a top level dressage horse.
For eventing I'd say he'd do well. I'm not sure how high he'd go, if your looking for a professional eventer, just because he is short backed, but I think you'd have a very good partner in him!
He is certainly cute, and seems to know his manners and looks to be an uncomplicated ride. Good luck!