I second the older horse thing. I am a big fan of been-there-done-that, older horses and they do have a lot of miles left on them. Age is just a number!
Is this horse going to be an only horse, or will he/she have a companion of some sort?
Not all horses can be kept alone and will do whatever it takes to break out and find another horse for company. I have a friend who has had her horse for well over ten years now and he still breaks out of the fence and goes looking for other horses! He's 25 years old too. My friend finally, after all these years, got the poor guy a companion.
If I was going to be getting a single horse, I would certainly ask any seller if their horse could be kept alone or has ever been pastured alone? Some horses can go berzerk when they suddenly get dumped off of a trailer at a new farm and look around and go "where is everybody"? It can make a seemingly calm and quiet horse suddenly nervous and skittish.