As you said the picture is not ideal, so I have no idea of his conformation. Of course when you go to look at him you'll want to check him out good. Once you buy him you most likely cannot return him, so be careful. Ask for his owner (or whoever is showing you him) to ride him around for you to see how he looks from the ground. Then ride him yourself and see how you like it, it might be abit rough in the beginining, but just take your time and try in get in sync with the movements. Check out his conformation (I would, but I can't see much from the picture) and how easily he walks, trots, canters, and gallops.... Look for any limping or issues that might be causing him pain, and then be sure to check out his hooves for any issues (thrush, puss, nasty frog, or maybe rocks/nails embedded). If you know how to check a horses age by his teeth, than do that and be sure that the owner is truthful. Check out the entire body of the animal, eyes (discharge, puss, redness, swelling, or cloudiness), ears, mouth (sores, etc.), forelimbs, barrel, groin, hind limbs, and around the dock/tail area.
So by now, I've probably overwhelmed you! If so I'm sorry, just be careful... You never know what you might find if you check the horse over all the way! Before you leave the yard, ask the owner about the horse's history, if it has ever had any problems that required treatment, and for vet records if he has them on hand. If you want to have a vet check the horse out, that would be best. But it isn't required! Also, 15h isn't very tall in a horse, so it'll depend on your height and how tall you want him to be. Be sure to check out that as well!
Anyway, I hope this works out for you!