He's a nice looking boy, good coat and decent muscle build. If you're going to start slow with just trail riding and light dressage I'd say he'd be fine for that.
Conformation-wise: He initially looks like he has a large head, but on second look his head looks like it fits his body. The eye then looks at the neck. His head looks larger due to his thin neck. That is something you can work on in your dressage lessons, just let whoever you take lessons from that you want to work on building his neck.
As for the fetlock on the cocked leg, I would definitely take a look at it if/when you go look at him. Have the owner walk and trot him and look for any sign of him being off or lame or any jerky bobbing of the head. If you have the slightest doubt make sure to mention it to your vet for them to check out when you get him vetted.
Also, the muscling on his hip, above his tail base, looks weird, but that may be because of his cocked leg which is throwing the angle of the picture off. He seems to have good strong legs for his build.
Overall, a nice looking boy!