What are the chances of my daughters father getting custody if he's not been involved in her life since she was born?

He may convince the mediator or judge that he is entitled to joint legal custody (decision making ) but it is unlikely he would get primary custody .or a high percentage of physical custody. You would most likely request reunification if your daughter does not know him or supervised visitation. It is also likely he will blame his lack of involvement over the last years entirely on you.