I, personally, know the woes of a horse you're leasing being for sale as well..Especially when you go to one of your first shows on the horse and the owner tells you that there's a few potential buyers wanting to look at her and watch how she runs, and you don't find this out until you're going into your pole class. -.-
If I were paying to lease a horse, I expect to have that horse available for me to lease (from the SAME OWNER) for as long as we agreed I'd be leasing. ST's currently for sale, but it's not advertised anywhere and I'm only leasing her until fair when I leave for college..so I'm not too worried about it.
Previous BO that I was leasing Lucky from put a for sale ad up on her without even telling me, my friend that wrote the ad and another friend that just so happened to come across the ad told me, and then BO told me almost 2 weeks later and that someone was coming out the week after to look at her.
I know how you feel when you think the horse is going to be sold right out from under you, and I'm going to have to say to just go with your gut feeling about the owner. If the owner seems like she -needs- to sell the mare, I wouldn't do it unless she just wanted a few more miles so she stayed in shape throughout her sale, but I would not pay for that. If she doesn't look like she -needs- to sell the mare, I would try talking to her about how you're worried about her selling the horse while you're leasing and you ending up horse-less. I'm sure anyone that's leased before would understand where you're coming from, because it is a worry.