When Ian was born, I said that I would not have another baby. Mostly because, like you said, I was so focused on the fact that I was lucky enough to have one and because everyone else told me that, too. I just kind of accecpted that for a while. Then, as he got older, probably about a year, I really started thinking about wanting him to have a sibling. I have 3 siblings and I just couldn't imagine not having them or Ian not getting to have one either. And, of course, there was that baby itch! So we started talking about it.
My husband did not want to chance my health at first. We had many discussions about this. My doctor was not exactly thrilled about the idea either (nor was he excited about the first one, but was supportive once I made my mind up to do it). To be fair to them, I had not always been the most compliant patient in the past, so I understood their concerns. I also had to be honest with myself and decide if I could really handle two kids. As we know, it really is the day to day of caring for them and ourselves that is the challenge of being a mother with cf. Another concern DH had was the fact that it took us a year and a half to conceive the first and he wasn't sure he was up for the disappointment roller-coaster again. He knew how much I wanted the second baby and we decided to try for one year and then know it was God's plan if it did not happen.
Originally, I had thought I wanted my kids closer together in age. My husband always said three years apart would be good (like he and his brother are), but I wanted them to be closer to two. I sure am glad someone upstairs had a different plan for us! I don't know how the age difference (4 1\2 years) will affect them growing up, but I am glad for it now. Ian is old enough to be self sufficient with a lot of things and that helps me care for the baby. He truly loves her and hasn't shown any real jealousy towards her. Since he starts kindergarten in the fall, I know I will enjoy the time I'll get to share with Isabella, just as I loved that time alone with Ian when he was a baby. While I hope my kids are close to each other as they grow up, I have learned with my siblings that age does not matter to the relationship once you're an adult. At least that hasn't been the case for me. My sister is 12 years older than me and is one of my best friends and I know she feels the same about me.
Oh my.... I have rambled! But to try and address all of your questions: I think the biggest challenges of having two are the same as all parents face. You want enough time for each child so they feel loved and special. You have to adjust your schedule to get everything done in the day, and for me personally, getting the rest I need isn't as easy as it used to be.
The biggest reward is seeing the way Isabella lights up when she sees Ian and how cute he is when he talks to her. They are building the relationship that I was afraid he would not get to experience. I know that they will always have each other, even when DH and I are no longer around.