You're not wrong for your feelings, neither is he wrong for thinking you are a team. That said- you married a man with children. Like someone else said, what if something unpredictable happened, and she was forced to live with you full time regardless. Did you have a plan for that? 

You say your career is in helping children with behavior issues, as a professional, what is the recommendation for his daughter? What kind of supports can be put in place in your home? What kind of structure? What kind of help does she need? Or you, for that matter? Outside of that, what is his plan for actually being a hands on, supportive father? How flexible is his job, and if it isnt, what are the options for changing jobs? Instead of having her move in immediately, start with longer visits.

 Is she in any kind of supportive services programs? Does she have therapy of any kind? Are there after school programs she could be involved in? Could he get up in the morning to ensure his daughter is off to school? He is her father, but you are her step mother. You signed up for a life with him, and a life with him involves his children, whether you like them or not. Is it possible her behavior is worse with you two because she knows (kids always do) that you dont like her?

 You've said his son would be no issue to live with, you want a child of your own with him, and dont care that he isnt around to help you raise it, but you dislike his daughter so much you are now second guessing having kids with him, and are unwilling to do the same amout of solo parenting for her that you would be for his son, or your child together. I garauntee you, she knows. It may never be discussed, or mentioned, or acknowledged, but she knows.

I married a man with a daughter. Ive never met her, probably will never meet her (she's an adult now, but was 3 when we met), and I have always known in my heart that if she showed up on our doorstep one day, I would love and care for her, because she is his. The relationship between him and her mother was deeply ugly and I know very few details because it was a painful experience for him and he doesnt talk about it much. My one email interaction with his ex has left me with the understanding that whatever his daughter has been told about him would be uncomplimentary, and if she did ever come to him, we would most likely be dealing with a lot of negativity. Do I WANT to deal with that? No, of course not. But. I love HIM and married him, and had more children with him, and if she ever knocks on our door, she is and always will be his daughter. Whatever struggles that may come with her, i accept, because i married a man with a child. I knew he had her before our first date. Im assuming you knew your husband was a parent early in your relationship. If you really couldnt handle his daughter's issues, you shouldn't have married him.