I have a 14.1 hh mustang that I used to jump 3', easy, with awesome form. I'm sure she could have gone higher, I just didn't want to. You won't find a horse with a bigger heart as long as you treat them right. My mustang came froma BLM Auction in Nevada. She has done everything from western pleasure, to jumper, to barrel racing and has excelled at it all. I have never had a better horse.
Buyer a horse from an auction is not cruel. If you are good a judging horses, you can get some nice ones from an auction. If you are not, take someone who is experienced with you. If you are talking about a BLM auction, don't worry about the horses left behind. They legally cannot be sold for slaughter.