I love working with fearful dogs. Fearful dogs are a lot of extra work and take large amounts of time. My GSD, Neka was very fearful when I first got her. It took me two years to get her to completely come out of her shell. But the day it happened, it was the best day of my life. I experienced the feeling of perfect joy!!! (think of your "best ever" day ... then multiply that by 10). It was incredible!!!
OK, back to reality. It sounds like she is starting to trust you so thats a good sign. You will need to work with her every minute of every day. You will need to expose her to everything in a controled enviroment for quite a while. Even something as simple as a car door closing or dropping a napkin on the floor can cause fear.
An older, calm and confident dog can be a great help to a fearful dog. The older dog can be a great mentor and dogs learn from other dogs much faster than they do from humans. But you must realize that you have to introduce them while the new dog is fearful. This CAN be a problem. You may want to introduce them in a nutrial location to see if there are any issues.
The most important thing you will ever do for this dog is to create mutual trust and then a bond of friendship with her. If you get her, plan to take a few days off to spend every moment of the day with her. Id go so far as to throw a sleeping bag on the floor and spend the first 24 hrs on the floor with her. During these 1st few days, you need to watch her, and see what she likes and dislikes. She will also be watching you!
And remember, you need to do everything in a positive and non threatening manor.