I personally wouldn't bother with a saddle. It is harder for a horse to gain correct muscle while only worked in a saddle (the compression plus the fact that a lot of horses tense up while working, which ******s muscle growth). I'd get a nice bareback pad and use that to ride in. Are you planning on showing her? If not, why bother spending a ton of money on a special saddle that may or may not fit anyway? Bareback is usually more comfortable for the horse anyway. You can do whatever you want bareback. It isn't any less 'safe' than with a saddle. (I tend to fall off more with a saddle anyway.
Do realize that the older she gets, the more likely it is to start sinking even further. Once those muscles get weaker, her back will go down. This can lead to arthritis and pain. I'd be sure to have her on MSM (super cheap, like $6 a month. Make sure you get 10,000mg per dose. It works awesome on most horses, reducing inflammation before it becomes a problem). A good joint supplement wouldn't hurt, if you can afford it, too.
Try and do exercises that increase muscle strength in the back. Trotting over raised poles while free lungeing is a great way to encourage her to lift her back. The emphasis is on prancing slowly through the poles, not racing through them.