I take in all sorts of rescue horses and project horses. In doing so I have come across two girls who have very similar stories to your story here. For whatever reason, they had to give up their horse. And I can tell you from personal experience when these girls hound me years later about wanting to know where "their" horse is, wanting to buy him/her back and wanting the new owners contact information... I do NOT give them the new owners contact information. I DO offer to give the new owner of the horse a heads up that the former owner is looking and "here is her number, call her if you'd like". But in both cases the current owner did not call and this made the former owner very upset, mostly with me.
Like it or not, Speed has brought around some very realistic questions. These are the things that I have told the other girls looking. He is no longer your horse and the current owners may just want to keep it quiet... for somewhat obvious reasons. I wish you luck in finding him and finding peace but be prepared to find out that someone does not want to part with him. Also understand that no matter how long you'd owned this horse, he is no longer your horse. You have no claim to him, so keep calm and be happy with any information you find - negative or positive.
Best horse in the world or not, I am assuming that if you owned him for ten years and you have been looking for another eight years (which still does not make sense to me that you haven't found him but you know that he has changed hands four times since your father sold him?) that this horse is well into his twenties by now? Then yes, you would be passing up a lot of youthful, usable, also nice horses by spending a lot of energy tracking down a horse that the current owner isn't likely to sell or he may no longer be around all together.
If you plan on saving and helping animals, why are you waiting to find this horse? There are many, many horses who could use a loving and devoted owner right now.
Any way, best of luck I suppose!