Well, learn as much as you can about his past. See, how he spent those 18 years will have huge bearing on long he will be sound to ride, and how sound he is "upstairs".
National show horses can often be a pretty hot horse. But, they can also be really fantastic show and trail horses, with huge stamina. I met a man the other day out trail riding his NSH , who he had kept in Alaska for years. The horse was ridden out on the trails with bears and all kinds of wildlife a common fact of wilderness riding, and it kept it's head.
Have you ridden him? Why is he the last choice? I did not read the "saga".
He has a lovely head. I am riding an 18 year old Appalousa who has tons of go. Arabs are known for their longevity.