You are making a lot of considerations of what you will leave open for the new person. The hard part of this is that you can't. You have NO IDEA what this new person will want out of life. You discuss the idea that she may want kids. Well maybe she dies. What if she doesn't want them with you? What if she doesn't mind being with you two, but already had another partner? Does that mean she wouldn't even be considered? It sounds like, just from what we see posted, that you want a third that will have no other relationships. Someone open enough to poly to understand that you both have each other, but not open enough to be interested in dating other people. That is usually where things go horribly wrong.
Me: Late 30s pansexual poly.
DH: My husband of 19 yrs and father of 3 teen girls.
DC: LDR of +4 year