OP,
This whole situation has clusterfxxk written all over it.
There truly is NOTHING in this situation that benegits you, but boy does it benefit him.
Like so many many men that have young children they just love to line up another woman to share or take the load from them.
This will change your home for ever.
This will change your daughter's home for ever.
You both enjoy your peaceful home, why would you want this.
There is NOTHING wrong will loving seeing the back of him and his children.
I love having family occasionally to visit.
We have a great time and enjoy the company, but after 2 nights I have enough and want my home back.
I think a lot of people feel like this.
You really shouldn't force yourself to do something so huge to suit someone else.
He is pushing this completely to suit himself.
This 100% is not in your daughter's best interests.
How would you feel if your daughter turned around and said she wanted to spend more time at her Dad's as her home has changed so much?
Would you also be happy doing playdates for the girls with their friends?
Your daughter may want to bring her teen friends home.
The key thing is that cute as his children undoubtedly, they are not your children and you would be dropping a bomb in your daughter's home AGAIN.
How he behaves now will be important.
Don't be surprised if he ends things.
He is after all primarily looking for someone to share the load with him, preferably with a ready home.
I don't believe this is in his girls bests interests either.
He is the one who will benefit most.