Edit: the post below assumes the sperm father doesn't want to be in the kid's life or the mother doesn't want him in her life. If he wants be a part of his daughter's life then things get even more complex, but I stand by your need to commit to both mother and daughter inthe face of a commitment.
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I had the EXACT same situation @25 as you, to the letter. 3 yo daughter with awesome girl I was totally falling for who lived at home, everything. Understand this; If you two get serious she's gonna want to move in with you, which will put you in a fatherly position even if you don't accept the responsibility. You cannot just back off and let her handle everything about the child-it will eventually drive a wedge between you and keep you from getting closer. I mean, you go out to IHOP and the kid starts acting out while Mommy is at the breakfast bar, what are you going to do? It's pretty unavoidable to be a part of her life unless you keep it at arm's length and never commit/ move in together.
If you want a commitment, you will have to make a decision to take the child as a part of your mate and part of the relationship, and be willing to be a father to the child, unless she expressly doesn't want this. Talk with her and define the boundaries she wishes in your dealings with her daughter and if she wants you to be Daddy, or take this time to tell her you don't want to be. Since her kid is female you'll need to think of her also when getting little non-sexual gifts for your gf like flowers. BTW this also includes Valentine's Day. Woe to you if you don't bring her some chocolate too!!!!
Even if you don't want to be Daddy you will still be an adult authority figure in her life and you will have some impact on her, and she will look up to you and may een come to love you as children don't care about particulars when loving someone.
Once you all get a place you will have lots of real-world problems she currently doesn't have such as rent, car payments etc to handle all on her own. Not saying she's immature, just saying what I went thru. Romance cools off quick when you throw in finance and things get tight. If you think you don't want the Daddy job I would not enter into a commitment with this girl, but conversely if you do opt for the instant family undersand the impact breaking up the relationship will have on her daughter. It's a huge question, one I agonized over for a few weeks and several talks with my gf, and "test" interactions with the little girl who is now my daughter. I was a guy who hated kids (still do in many instances of shitty parents) and never wanted one. Now I like being in the relationship more to be a dad to my daughter than a lover to her mother. Good luck to you.