IMO, I'm with the crowd that says do not buy an old horse to sell it later. That is not fair to a horse that has been spending all its life serving people. It deserves to be comfortable with a steady environment, an owner that is willing to keep consistent feed and care until the end. I sold my fathers 20 year old horse after I went to college, I couldn't afford to keep both Jake and Jessie, Jessie would have had to live in a pasture by himself with no company, which is no life to retire into. I sold him to an older couple that only wanted a trail horse to cruise around on. They promised that they had no intentions to sell, that even if they couldn't ride they would keep Jessie until he passed on, and he would always have a buddy. That was best I could wish for the horse that had honestly taken care of my family. I know people lie, but I even inspected their property, checked on vets they would use and examined their horse knowledge. Now, Jake will stay with me until he passes, I already have retirement plans set up for when he is either physically or mentally done with being rode.
Full Leasing an older horse would be a great option for you. That way you don't have the commitment past a year, and you don't have to worry about selling later on.
Older horses should live out their lives with dignity and stay put