Oh no, I'm sorry to hear that
I had a horse have a clean pre-purchase but failed tendon palpation. I sprang for ultrasounds and they found a whole mess of issues and holes and tears that will plague that poor horse the rest of his life.
I loved that horse and I wanted it to work out so badly I was willing to accept a lot but that was a deal breaker. He wasn't even lame at the time it was so difficult for me to come to terms with.
I do regret doing the ultrasound now though and should have just listen to my vet during the prepurchase. Would have saved myself a lot of money, time and heart ache
If it would make you feel better to do the x-rays and you have the cash to spend, go for it. If the vet seemed concerned, it's probably not a good idea. Sorry to hear that.
Take it from a girl who rescued a horse with known lameness issues who failed a vet check... it's endless vet bills, sadness money and trouble. My gorgeous wonderful sweet little quarter horse is now in the pasture board after having a lameness relapse :(