We always thought we wanted a big family. Then I had a similar experience to you - SPD, PND, reflux, and DD just wouldn't sleep. From quite early on, we swore she was going to be an only child. I felt terrrible about it but just couldn't face doing it again.
When DD was 10 months old, she slept properly for about 2 weeks. We felt so amazing after catching up on the lost sleep that we decided to go again and conceived DS. As soon as I was pregnant, DD stopped sleeping again. In the end, she didn't sleep through till she was two, and DS was the same. He had very severe reflux so his baby stage was much more stressful than hers, the hospital were constantly telling us he wasn't feeding enough so not only did we miss out on a lot of sleep but many of my waking hours were spent in tears because I couldn't get him to feed.
It was fairly hellish at times to be honest. I did catch myself thinking that if I'd known it was going to be like that, I wouldn't have had kids.
They're 4 and 5 now, and if I could go back and do it again, I wouldn't change a thing. They have great fun together, they learn so much from each other and they're very much a team. For her sake, I'm very, very glad that we bit the bullet and did it. Plus he's the most sunshiney little person you could ever hope to meet and makes us smile so very much - we wouldn't laugh half as much if he wasn't around.