I would be leery of an *unbroke* half-sighted horse, especially if it's to be a kid's pony (although, maybe this one isn't for kids, I dunno). Even if the horse is well-adapted to the lack of sight on the one side in its life so far, it is kind of a tossup whether problems will develop when it's backed and trained.
If the horse is not spooky or crooked, there is not necessarily any reason it can't be a perfectly good dressage mount (although IME one eyed horses can be a lot harder to supple and straighten, b/c they often get used to carrying themselves crooked, moreso than normally sighted horses). Certainly there are any number of good one-eyed dressage horses out there, although they tend usually to be horses who lost sight *after* becoming fairly well trained.
Jumping is iffier. Even for horses that were very experienced hunters/jumpers/eventers before losing the use of an eye, many are not as confident or safe thereafter, and it is questionable how safe it is to jump a one-eyed horse at speed (e.g. cross country) or large fences, even if they have lots of mileage in the sport. I dunno 'bout the wisdom of trying to *start* a one-eyed horse over fences, at least not for more than just popping over logs and crossrails type of thing... I have never done it, and am quite willing to believe that an extremely level headed and coordinated horse could learn to jump with only one eye and be reasonably safe over modest fences; BUT not all horses are like that and the potential for accidents and/or Scaring The Horse And/Or Rider Pretty Badly is much higher if only one eye is working. 
I guess, to me, I would not consider buying a one-eyed horse as a prospect for any sport that involved jumping (perhaps you are considering eventing?), and an unbroke one eyed horse would be a bit of a gamble for any flat riding either. Certainly you'd want to assure yourself that the horse is absolutely dead-level-headed and natural-born bombproof, and does not move crooked (head cocked, neck carried to one side, leaning on one shoulder, that sort of thing)
Good luck, have fun,
Pat