I think I have a "success" story if that's what you are looking for? Our kids were 2-5 so on the younger side which I do believe makes a difference in many ways. It does depend though on the background they have, no matter the age.
I think the main thing to remember is that even with challenges, if you are honest with yourself about what kind of challenges you will/will not be able to handle, it makes a difference. Going into it with the acceptance that there will be some adjustments to go through and some challenges to face makes it "easier" too. (rather than the "we covered all our bases and everything will be perfect" train of thought)
I think I would adopt a child younger than your dd, just my opinion on that.
My kids are 8-11 now and while there are still some small "holdovers" from their early years that can come up here and there, and my oldest boy is going through some things a bit more as he gets older, I don't feel it's so different than any other family. We participate in sports, school activities, family things etc. and nothing we deal with prevents us from doing these things.
Talk to people through your agency too...most should have families they have worked with prior that will be open to sharing their experiences with you.