Californians, being one myself, have a tendency to try and take advantage of the horse market. I would not buy that horse unless you had him vet checked and ridden by a friend who knows a LOT about horses, or you may very well get yourself into a position that will cost you a lot of extra money. Just from the pictures, I can see that he is a little short at the knees and a little bit over. This is just going to make him a little bit clumsy and make him have a shorter stride. So he might just have to work a little bit harder and that's why he's sweating. Sweating isn't necessarily bad though, as long as he's not breathing too hard he should be just fine. If you want to work on his stamina, I would start with 20 minutes of forced exercise at a trot once a day every day until he is no longer breathing hard at the end. Then I would increase to 30 minute sessions, and then to trotting him up hills with a gentle slope, and increase the incline as he shows improvement. It's good to build up a horse with his conformation slowly because it will make him less apt to injuring tendons and joints. He looks like he would be a good match for you, but if he is not registered, don't pay more than 2,000 for him.. the way he is and the fact that he's a rental horse I would offer him 1,800 simply because I know he's safe to ride. But look around at different horses for sale in different areas, particularly Escondido/San Diego. You will find gorgeous registered horses that are the same age that you can ride that have had professional training and come with vet checks/ written guarantees of health and soundness. So don't settle with just one, if nothing else look just for the added experience!