I'm no conformation expert at all, but most quarter horses are downhill. Im not used to seeing ones that aren't lol He seems ok to me, not what I would breed to any of my mares honestly, but I do competition, so that would be important to me. Most reining bred horses are sickle hocked, its just how they are built. He seems to be a nice stallion and quite versatile.
Now the big question is, besides the exported semen, if they will breed to a grade mare. I went and looked at your mare (she's bloody adorable btw) and most registered studs, particularly ones with competition points and money earned, will only breed to other registered mares. In the AQHA breeding world, most will only do other registered quarter horses, but you find a lot supporting the Jockey Club (Thoroughbred), APHA and ApHC, it depends on the owners market they want for their stallion. It sucks sometimes, but they're picky about who their stud breeds to in order to gain more prestige for their stud, but as a breeder, I also understand that now. I've run into a lot of nice people that would breed to grade mares (I bred my arab cross in high school to an amazing racing quarter stallion), but it may depend on many factors. No harm in asking, but be prepared just in case. I would send an email asking on their breeding requirements, and see if its something they would consider. Good luck!