It sounds like you're blessed to have a loving father figure for your daughter. Because of your ex-husband's commitment to his sons, he'll always be a part of the picture—a constant presence in her life. It's important that you and he make a vow to keep him involved in your daughter's life and to not let new love interests (yours or his) disrupt that bond. She'll already experience some rejection due to her biological father's abandonment so it's crucial that your ex-husband stays invested.
He's already been more of a daddy to her than her sperm donor. This is a special guy with the ability to love not only his own flesh and blood but a child who's unrelated to him. Many men would simply say, “It's enough for me to take care of my two boys. I don't want to take on another child.” Don't take his involvement for granted and always express your deep appreciation.
Your daughter will benefit immeasurably from having him in her life. His devotion will make her feel worthy and build her trust in men. Their bond will be the foundation for her future romantic relationships. She'll look for guys like him, who are responsible and caring, rather than just any dude who satisfies her longing for daddy.
You should foster the connection between your ex-husband and your daughter and let him know how much you appreciate him. Sadly, our society today minimizes the importance of fathers and many men don't understand the enormous role they play in a daughter's life. They are the first intimate relationship a girl has with a man. They provide validation that she is strong, valuable, and capable. This contributes to her performing well in school, picking suitable guys to date and marry, and being bold enough to take on new challenges.
I admire your desire to do what's best for your daughter and to realize her need for a daddy figure. Best to your family!