My experience might be of some help… I purchased a 15 year old schoolmaster from a sale barn who came with a small amount of “baggage” and she was the best purchase I’ve ever made.
The sale barn/broker took a commission but it was off what the owner would receive, not me the buyer, so the purchase price was higher than what I would have paid going through a private sale. This barn also specialized in very high-end, expensive show horses so prices reflected that. I never spoke to the owner and was not given any of her details as all information had to go through the broker. My understanding is that the owner paid board on the horse until it sold, and the broker would take a commission on the final sale - so the higher the sales price, the more commission received. The benefit to the owner is that the horse was being trained, marketed, cared for, shown, etc by a high end facility with many clients/trainers without having to do any work for it other than pay upkeep. I have a hard time imagining a scenario where an owner would not be paying board and the broker would have to eat that cost, but I’ve only been on the buying side of this arrangement one time so I’m no expert.
As far as my mare goes, I knew really quickly that she was what I needed/wanted even though her age really worked against her (I was looking for a youngster). I was told she would likely need hock maintenance eventually by the broker due to her age and resume, but she was very happy and comfortable both times that I tried her. I think she came up 1 or 2/5 lame on the RF when jogged on concrete in a small circle during the PPE, but the vet was honest that because it was only showing on concrete in a small circle and she seemed so well-suited to me and my goals that it likely would not be a major deterrent unless I wanted to pursue it further, and yes be aware of future maintenance.
I chose to take the risk and bought her, as I felt the risk was justified based on her age, ability and what I needed. That RF flared up exactly one time in the 5 years I’ve had her, and it was when we boarded where the arena was just hard packed base. We confirmed mild ringbone and moved back to a barn with “normal” footing and the issue disappeared.
She is now 20 and has stiffness when warming up, some days more than others, but xrays show very clean for her age/resume and we have been happily competing at Novice/Pretraining eventing for the last ~3 years. The fitter she remains, the less stiff she is. We’ve come across the occasional mystery lameness but it always resolves itself, and even with great draft feet she needs front shoes in the summer or she gets ouchy. She’s had hock injections once and I’m going to try SI injections soon to keep her comfortable but overall she has been happy, loves her job and went over and above what I thought I wanted in a horse.
As far as warm up goes, Ingrid Klimke is a big advocate for a long, slow warm up: I audited a clinic with her 2 years ago and she mentioned at the time that there is absolutely no harm in spending a good 10 minutes in walk work if needed! That really changed my perspective on my expectations for riding.
I hope that is helpful to your decision, whichever way you decide to go. In your case, I’d definitely wait until the current acute lameness is no longer a factor before buying though. If you are willing to try a lease and can come to an agreement with the seller, make sure it’s long enough to give you time to have him sound a while and really be sure it’s a shoeing thing. If the seller isn’t willing to work with you on that, I wouldn’t take the risk.