Kids don't need to know the truth, they see it before even being talked to most of the time.
Like the rest said, you need to be a parent first, and have good communication with your daughter that you are there for her, when ever she needs you, and keep that promise, be the parent she depends on.
And yes if you are worried about her feeling abandon yet again, from a parent, go for primary or 50/50 custody.
I don't like seeing kids go threw a divorce, but a adopted child has so many scares before even being choosen, and I think it's terrible that a parent can't see that.
Good luck, and best to your daughter and her growth, give her more love then ever!
In three words I can sum up everything
I've learned about life:
It goes on.