I’ve run the gamete of lameness issues in the past 6-7 mo with my 5 yr old gelding with stifle issues (catching and secondary synovitis in stifle). One vet said hocks, one said SI, and my final vet who is lameness, sports med, and surgery, said stifle. Stifle is what I thought it was all along.
Yes, there can be lots of secondary issues that end up leading you on a wild goose chase in an effort to pinpoint the problem. We had multiple injections in hocks and SI with no improvement. Stifles x-rays and ultrasound also looked good, even though vet and I knew it was stifle issue. The xrays are only going to show bony changes that are typically quite far along, as well as any OCD lesions or cysts. Important to rule out though IMO. Ultrasound will help rule out any ligament tears or other soft tissue issues.
Since this is a trial horse, not sure I’d buy it. I was only 4 months into owning my guy when he started having problems, so the bulk of the time I have owned him in the past year has been rough. That said, loose or catching stifles is not the end of the world and can be a problem very easily managed/corrected with proper diagnosis and treatment. All depends on the horse, prospective owner, and what the horse will be used for.