To whoever said that he will love her if only she is well-behaved: That's just crap. Remember, it is the job of the parent to love a child NO MATTER WHAT they do (not meaning you OK their transgressions, but you damn sure still love them and make sure that although you are angry, hurt and disappointed, they can still count on your love).
As someone who is 30 years old and still haunted by scars so deep you can not imagine because of a step father who made everyone believe that he loved me and my brother like his own while really he just wanted to dominate, control and hurt us, for whom nothing was ever good enough, we were NEVER "obedient" and well-behaved enough, no matter how hard we tried, and tried we sure did, while nobody, not even our mother, believed us - only our grandmother could see how we became more and more damaged each year, and she was powerless to do anything but love us and try her best to talk sense into my mother, I can tell you this:
To this day, because of his abuse (which never left a physical scar, I hasten to add) I can not function normally, and have to reside in a mental care facility from time to time - and that's just the tippy tip of the ice berg. Is your desire to have a husband so great that you will put your vulnerable daughter, who relies on you for protection, at risk of even a fraction of that pain, just so you don't feel a little bit lonely sometimes? Because that's what some parents do.
Watch how your to-be husband acts when he's with her, and try to sneak in and watch them when he DOESN"T know you're even in the house. Does he insult her, damage her psychologically, and then call them "jokes"? Does he wound her with his words or his hands? How does he talk to her when he doesn't know that you are on the premises? What does SHE say about him? LISTEN TO HER and call off the wedding if she says anything alarming, please, I am begging you.
Maybe your particular fiance will be fine with your daughter - I have friends getting married soon, where their 5-year old daughter isn't his, and he loves her to bits and dotes on her, and she loves him to bits, so it CAN work out, but you must listen to your child and act accordingly.
Further, when you DO marry, do NOT let the step-parent be a disciplinarian of your kids at all. Sorry, but that's how it's got to be - the step parent can have all the fun, all the love and all the hugs, but they cannot be the one wielding the (actual or metaphorical) rod, or saying when/how it must be used.
If she tells you something that alarms you, never make it off as "just a joke" or a "misunderstanding".
I hope it works out for you, and especially for your daughter.