Camelbak makes a woman specific pack that you could use for hiking and mtb rides. However, I would test it out at the store; you might want more storage space and a bigger hydration bladder, depending on the type of hiking you are doing. They make bigger sizes for men, but the straps usually hit you right across the chest and squish your boobs in very awkward places. Go to REI and test out all the ones you think you might like. I actually use mine without the bladder in it for a backpack when I commute on my road bike to work (I can't put any racks, etc on my road bike).
I can't help you too much with the purchase of a mtb. I am an "advanced" novice mountain biker. I have a full suspension mtb, but I just don't ride on the trails that often to get better. I do like it, though and it is a nice change from the road. Too bad I can't rip myself away from my road bike enough! If you are going to be riding mostly fire roads and easier trails, you can get a reasonably priced hard tail mtb.
Ladies, any advice on brands?