Can anyone help?

I am a father to a 6 year old son. His mother has not seen him in over four years and he does not even know her. My wife has loved him since he was an infant and she is all he knows as mommy. My ex has never been consistant in his life even when she was coming around. She would go months without seeing him and then just pop up. We never denied her visitations. She never would pay CS and when she felt pressure she would. I would say she has paid a total of $3000.00 his whole life. I took her to court but she didn't show so we were awarded everything. We should have tried to adopt then but it had only been a couple of months that she had last seen him. She has made no attempt to contact us and we have not moved nor have any of our numbers changed. She has not called, emailed or sent christmas/birthday presents or cards. She has paid NOTHING in over four years. Now, she wants back in his life and we want my wife to adopt him. We reside in south Florida. We always wanted her to adopt but we couldn't ever afford it and we knew no matter when we filed she would contest the adoption only to be a pain. My son is a possesion and nothing more. I think someone is pushing her but can't be certain. My son wouldn't know her if she came to our front door right now. What do you think our chances are at being succesful at a step parent adoption with contested from bio mom? Please know my son does not know the BM as his mother he doesn't even know her.