We have taken shoes off a crippled horse of ours, and after a few months of being turned away with regular trimming he came back sound.
If he looks uncomfortable we do not ask him do much, only what he is able.
It takes a while, but he came back to us. He could not walk when we got him!
My family just bought a little Welsh mare for my little brother, and she is seventeen/eighteen. She is absolutely fabulous, jumps like a star and has all the life of a horse half her age.
This mare sounds lovely, and as long as she doesn't have any serious illnesses you should be fine. ^_^