I see 2 very important questions that are missing:
1) Does your soon to be DH have any children ? no
2) If not, does your soon to be DH want a child of his own that he doesn't have to share with your ex? he def wants children of his own even tho he loves my son
Personally, I think those are the most important questions. If your DH has no other children of his own, and wants kids, I think everything else will work out. As far as your other questions specifically:
a) I can relate. My DD(5) is an only (not by DH and my choice, but by circumstance) and she absolutely DOES NOT want any type of sibling. If a sibling would have happened, she would just have had to deal with it. I'm sure she would have adjusted. I'm sure you and your new DH have plenty of love to spread around. You just have to make sure your DD feels special even if a sibling is added. I wonder if it is fair to my DD that she doesn't have siblings. I'm lucky she doesn't want them anyway. I wouldn't let your child dictate your choice about whether to have another child.
b) Is there a reason that is something you would even think about? Do you or your DH have some sort of family history of concern? 30 isn't old. It is not an age that as a general matter you should need to worry about things. You never know what any kid might need. You never know what might happen in the future to any member of your family, that might cause them to have special needs or extra attention. I wouldn't be overly concerned with "what ifs". I feel that if I don't have another child in the next few years I prob don't want to , I love my mother to death, but she waited to have me till she was almost 40, her health was failing by the time i was a teenager.. I would rather my kids not have to deal with that. ( if possible of course)
c) Kind of along with (b). Anyone can lose a job or get hit by a bus tomorrow. You never know what the future will bring until it gets here. If people thought about that, no one would ever have any kids.
I do think finances in general should have some weight on the amount of kids you have. If you can't support 5 kids, you probably shouldn't be having five kids. However, again, if everyone waited until they knew they were completely financially secure for the rest of their life, then there wouldn't be that many kids out there!
Good luck with your decision!