That is commendable.
I hope you both are prepared for the drama that will ensue due to the fact that you guys have a personal relationship with the mother.
My wife and I have thought about adopting, but it would have to be a situation where we did not know the mom, and there would be no other contact with her whatsoever after the child is ours.
If you feel the potential drama in this siuation is worth it, more power to you, and like I said before, your point of view is commendable.