I would wait until after your trainer's seen him. We went out to look at a horse for my 10 year old daughter and the first time she got on him, he was fine. But the second time we went, along with her trainer, he was very hot and much to forward for her. He had been in his stall more than usual, but still, he was a very different horse and I'm glad we didn't buy him. My point is that quite often, a second ride can reveal things that weren't apparent in the first. The sellers should be able to wait until you can get a vet out. Did you ask about a trial? With the horse we ended up buying, they wouldn't do a trial, but they did offer to buy him back within two weeks if he wasn't suitable. We drew up a contract and it put my mind at ease. Turned out he was a great horse, but I appreciated that they had enough faith in him to guarantee him in a way. We also ended up doing the vetting after the purchase (during the two week buy back period) so it gave us a bit of leeway.