Thankfully the Owners are trying to be honest BUT:
"the beginnings of arthritis in the butt area signals some sort previous injury to me.
My 26+ Arab was seven when I rescued him starving; he also came with an injured vertebra. Injured to the point where I would not be able to use him for the type of trail riding I did back then.
So he became a wonderful lesson horse for children under 12 and 100 pounds. He also turned out to be the best injured or sick horse nurse maid anyone could ask for.
My 17 yo TWH has a fractured sacrum from the day he decided to catapult himself backward off my 4- horse trailer.
Both horses see a chiropractor regularly and are on chondroitin/MSM.
Hopefully you can at least see the papers on this horse to see if it's Impressive-Bred and therefore might also carry the HYPP gene. That would really compound your rescue effort
Much to think about and the answer from my perspective is "yes, I guess not". In other words, if you've got plenty of horse experience and savvy, go with your gut when you look at the horse.
Best of luck on the outcome