Abigail may not be completely unique, but you definitely don't hear it every day! I only know one little girl named Abigail at the moment.
I do prefer Abigail over Sadie, but Sadie is a cute name and probably less popular.
LOVE the middle name you have chosen. Isabella is a classic name and while it's gained popularity, I wouldn't worry about that with it being a middle name.
If you love Abigail Isabella, you should go for it. Abby Belle would be a cute nickname, I think