This doesn’t sound like a $50k horse. If you love the horse and don’t care about showing, it could still be a great fit for you and your goals. But I would try to negotiate a suitable price that reflects the horse’s major holes. If you hope to make it to the show ring, I think there are huge red flags here, and you need to be prepared for the possibility that this is a horse that just doesn’t wish to participate in horse shows. Is that a deal breaker for you?
One thing to think about is what you could sell this horse for in a year if you want a show horse, and it still will not jump around a course at a horse show. Horses are never investments, and any horse can of course become instantly worthless. But I think it’s a good way of thinking about the market value of this horse with the information you have vs. just thinking about how much you love the horse and what you can afford to spend.
We all hope we can improve horses in our care. But I think it’s often best to assume the horse you are buying is the best it will ever be. Right now, you are buying a horse that has declined to complete course after course at shows in the United States, with your trainer overseeing its management and preparation. You have described yourself as an amateur who makes riding errors. Does it make sense to overpay on the assumption you can fix this problem in the same program as the rider who struggled with it?
I agree with others that if you felt ready to show yourself, doing some classes in a real show environment would be the best way to really test the horse’s willingness to jump for you. And the trainer might be right that it was really just a rider chemistry issue! But if that’s not really in the cards right now, then you probably should assume you are buying a practice horse, not a show horse.