Hi Emberbit -
Certainly I don't know that there are really any "off limit" questions. Well, okay I changed my mind - questions regarding their sex life would be off limit - really I don't know why you'd want to know, but anyways.
I think those questions are fine to ask, although I would tread lightly when asking why they can't have kids. I never did actually ask my son's parents, but it did come up in conversations. For me I wasn't overly concerned as to why they couldn't have kids, just if they were planning on having bio kids.
I think it's important to see them with their other kids and get to know those kids as well. On our first meeting we met only as adults to get to know each other, when we were sure we wanted to match then we started including their 4 year old daughter. I am not sure how they explained who we were to her, but it worked well.
It was so good to see them with her and how they interacted. It gave me a sense of peace to know they really were good parents.
I wish I would have asked about plans for schooling - my son is in French Immersion which I am totally okay with, but I never asked about it.