I think she sounds perfect for your family and Duke. Obviously she needs training and the previous owner admitted he didn't have time for it. So it's not that she's untrainable, she just doesn't have someone to work with her.
As WELOVEPUPPIES said.....follow your heart. Only you will know if she is right for you or if it is the right time. I think the fact that you were wanting a small dog and this one came up means something. Perhaps Rocky send this girl to you.