I might have missed the answer to this way back in the thread, so I apologize if I'm being redundant. You mostly plan to trail ride this horse? I'm assuming that you will primarily be walking and trotting, and little or no jumping? If so, are there any riding schools around you that you could ask if they have any horses that need to step down to an easier life? I got my old TB that way. He had arthritis from racing and couldn't handle 10 hours of lessons a week any more. He could handle what i needed him for, however. I was just getting back into riding after taking a break to have my girls. I just wantef something well-trained and quiet that could do 3 or 4 moderate rides a week. He was just the ticket. Fabulous horse. He had been a very successful steep chaser and hunter/jumper, and had been a staff horse for one of the local fox hunts. No horse is bomb proof, but I really never had to worry much with him. On the rare occasions he did spook, he would hop a couple steps to the side and then practically blush with embarrassment!
Similarly, there is a lovely horse at my training barn that actually competed at the Rolex horse trial in his youth. As he got into his teens, he was no longer equal to world-class competition like that. But he was far from ready to retire. He now happily hauls a junior rider around the lower levels of horse trials, and seems to relish every minute of it. (i'm sure the current owners paid a tiny fraction of what he was worth in his heyday) Everyone likes to feel useful, I suppose. Even horses.
Try to find something that is currently in work, even if it means you have to go a little older. PP are right, nobody doesn't ride a good horse. If it has been sitting in a field unused for a year, something is wrong. Is there any way that you can take your trainer with you? I know you would have to pay a commission, but it would be a lot cheaper than in the long run than ending up with a horse that belongs in a can. Trainers also tend to know about good horses in the area, horses that don't ever show up in the sale ads.