I would say you should pass. Mainly because you have no intentions of giving this horse the life she deserves, and are already planning on passing her on. No horse comes with guarantees. You could go out and buy a 5 yr old that develops Navicular tomorrow, or gets injured. Age has nothing to do with it. THere are also many younger horses that need chiro regularly, or supplements or both.
No horse is perfect. Until you are willing to find one that you know its faults and are willing to accept them (and they you....
) please lease. Horses are a huge committment, and not like a car that you trade up every couple of years for a new model.
I also have an old guy who is great, but started having issues and requiring supplements etc around 20 yrs old. He is now unrideable. That does not mean he is any less vauable to me. He raised my daughter, taught us both a ton and is a member of the family. Even the non-horsey men of the family feel strongly about this horse and we will all be crushed when his time comes.
This horse deserves better.