SO wants to have a child with me

So we have been together for a year and a half and living together for 6 months.  I am divorced with two children, 6 and 10 years old.  He has a child as well who is 9 and she lives with us.  We talk about getting married, which I am all for....but he wants another child.  I'm worried about having another child for many reasons....having to start all over again with diapers, daycare expense and all other expenses, etc.  I feel like I'm over that feeling of wanting more kids especially after my divorce, which took a huge toll on me and my children.  On the other hand, it would be amazing to have a child together.  I love him with all of my heart.  What do you ladies think?  Has anyone been in a similar situation?  Thank you