I was 38 when I moved in with my girlfriend and her two daughters (2 & 6 at the time.)
I've been living with them for about 18 months and it's a tiring but rewarding life (the difficult bit is when they want to pull you in opposite directions - both insisting I spend more time playing with the other! So your guy wouldn't have that issue :) ) I very genuinely love them, not a father's love I guess but I've been lucky enough to be fully accepted, they see the four of us as a family and that brings its own type of love.
However, their dad is still a feature and has them ever other weekend. He was a lousy dad when he lived with them but does a great job now (better than many of the Disney dads I read about on here I think!) Ironically since he left, they've finally formed a fantastic bond that children should have with their parents, and it's wonderful seeing how content they seem when they come home to us.
But it's difficult, really difficult, to put up the emotional blocks required constantly to step that little bit back and not step on his toes. I talk about him a lot, positively, when they're home so they feel comfortable and feel his presence here isn't taboo or anything - even though at times it hurts doing it. Trying to give them as much as I can but holding back just enough to be clear that I'm not 'trying to be dad' is a constant stress.
So, that's literally the only difficult thing, and your guy won't face it. I envy him.
As for advice - don't worry about the word 'love' - it's a concept that every person defines a little differently. There's only two points to tick here:
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Will you daughter's life be better for him living with you? Will the way he treats you be a good example for her growing up? Will she feel like she's gaining something rather than losing a bit of you?
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Will your partner enjoy the life? To be honest, I think us guys in our late thirties that find this hit the jackpot a bit. It's a point where we want a little more.. fulfillment? in life and bam! Ready made family. That probably sounds terrible in a way but I when I come home and have two kids run to the door to greet me enthusiastically, when I can remember being a single guy not so very long ago, makes it feel like a lottery win. So hopefully he's at the same stage. Like I mentioned earlier, I would have loved to ease in gently with one kid first sometimes - two is going straight to hard mode!
A lot of the elements are in your favor I think, the age is young enough to be accepting, no blending etc. Don't push the 'love' - my girlfriend wanted the same in a way (not a dad, but a family unit that did the things she'd dream of - go out on trips as a family, instead of one staying home on the sofa every time etc.) but I don't think I can love them the same way. I'm happy that me and the girls have something else that is our own thing. It's definitely love to me. But I could have been the kind of guy that is a little tense with the word 'love' and hesitant to describe it as such, but it would still be the same thing, if that makes sense?
He needs to be something fun for her to begin with. Her feeling when he comes round or you meet should be that it'll be fun. No kid is going to react well to having a new parent forced on them.
And if he starts being there a lot / eventually moving in - whenever you feel that she craves a little 'just mummy' time make sure she gets it.
I hope it works out :)