I rescued a young gelding and an older mare. The mare had arthritis because she was older and my gelding had arthritis because of any old injury and not being worked in about two years. As soon as I started working them both their arthritis decreased majorly. They still both have a little bit of arthritis but the work has really helped them. I don't have to give them any supplements or medications for that. If you give the horse consistent light work it should help.
When a horse has arthritis it just gets worse if they stand around a lot. If you really love the horse you should buy it but you should just realize that her work will be limited as it is and especially if it gets worst.