My daughter's biological father has just let me know that he wants to voluntarily give up his parental right to allow my husband to adopt her. If he states that in a letter and has it notarized will that be accepted in court?

If your current husband is willing to adopt the child, then the courts may allow it. Try filing in court rather than sending in a notarized letter because ultimately, it's the court's decision as to what's best for the child.

We want to get this letter and then go to the lawyer and file in court with this letter.

If everyone's in agreement, a lawyer isn't necessary. What state are you in? A lot of states have pro se forms so you don't need to get an attorney.