Read other letters about this article
Assuming this is a real letter (and from my experience... as fantastic as it is it COULD be) - the problem doesn't lie with the prospective stepmom, it's the Dad.
No one will be a suitable stepmother for this child until the father changes the way he interacts with the child, and rethinks what he is willing to do to help prepare his child for the real world.
Unfortunately, given the assumed age of the man and the stated age of the child little is likely to change.
The goal isn't to aim for perfection, but manageability. It is unreasonable to expect any stepmother to raise a child with an uninterested biological parent. It will never work. An ABSENT biological parent, maybe, but not a present and uninterested one. Different dynamic, different demons.
Folks, stop blaming the potential stepmom. Any sane person would feel this way. Blame the father, the mother, the stepdaughter, the nanny and the other enablers.
Dad should have stepped up long ago and put a different structure in place for the child (as much as was legally possible), and if he couldn't he should have paid someone to do it for him once she was in his custody.
A mentally ill mother is... well, what it is. You think of the good things and try to reconcile the bad and hope for the best.
A loving but enabling nanny does what she is paid to do... Obviously, Dad should have picked another one.
The daughter is a child, but even children must be responsible for their actions (or inaction). Children are NOT stupid, and to assume she's an innocent is blindly stupid. Even if she has development issues, it doesn't mean she hasn't contributed to some of her own problems. And she's probably playing all of you more than a little bit...
And the potential stepmom - plays the cards that have been dealt her. The good news is the child is almost 18 and at that point there will be changes in the relationship with all involved. Perhaps THAT is the right time to start living together or get married.
Right now you are entering a losing situation without any hope of affecting change. The change must come from the father and the daughter must meet him part-way. If neither is interested in change, then just stay out of the way til she's an adult and out of the house.
Of course, in the real world, the "wait" solution rarely works. And it is unfair to most involved because it is asking people to put a hold on a basic animal instinct - mating. And the bitterness the wait can cause can be very detrimental to an adult, and the ensuing bitterness can color the relationship with the child.
We all try to do what we can... I think. Sometimes it works, sometimes it doesn't.
So = if you want to continue as-is I suggest counseling for Dad so he can learn how his lack of action with his daughter will affect his relationship with you. And then perhaps move on to counseling to help him become a better parent. And then counseling for the child to help her adapt.