Agree that this looks like an SI injury. I would not buy this horse.
Been there, done that, would not recommend. It’s a ton of work to keep a horse like this fit enough to support their SI, if they stay sound at all. I had a horse with an SI injury (and a roached back). He was retired from jumping permanently after the injury was discovered. We spent a ton of time and money rehabbing him - massage, chiro, hills, pasture turnout, injections, careful work with a skilled rider - and he never really recovered enough to be a riding horse.
SI injuries do not mesh well with lessons/novice riders - these horses need to work properly through their bodies, all the time, in order to have enough support around their SI joint. That’s (in my experience) something a lesson program can’t really offer. I’m also not sure if this horse would stay sound jumping even low level fences, since he’s not jumping already.