I see that lots of other people have already responded, and I'm not really sure if I have anything new to add, but I'll throw in my 2 cents worth anyway.
I am an only child. As a child, people would ask me all the time if I wished I had brothers and/or sisters. I was happy and always said "no." I spent tons of time with my parents, had lots of unique opportunities, and a fantastic education. Some of that might not have been possible with siblings. I had (still do, not as good) a wonderful relationship with my mom growing up -- we really were best friends for many years. Now, that I am older, I realize how completely inappropriate and bizarre this question was -- first, because I couldn't change it even if I wanted to, and second, because the reason that I was an only child to begin with was that my parents had trouble conceiving and despite their best efforts and all the interventions that 1970 had to offer, I was all they got.
I first decided that maybe, just maybe, siblings might be nice to have when I turned 24. My oldest childhood friend lost her father that year, and it dawned on me for the first time that once my parents passed away I would be "alone." Since that time, I have gotten married, (to a man with 4 siblings!!!) and we have 3 beautiful children. Fortunately, my parents are alive and healthy and able to enjoy their grandchildren, but when they do pass away, I will not be alone.
Three children is not easy, but it is very rewarding. I, too, had post-partum depression. The first pregnancy was the worst, because PPD sneaks up on you slowly. All the books and magazines say that it is normal to feel moody and tired. Plus, your life changes radically. I went from being a practicing attorney working long days downtown in an office filled with adults to being a stay at home mom with virtually no adult interaction all day long. It wasn't until my first was 6 months old and I was borderline suicidal that I got treatment. Fortunately, I responded well and was feeling like my old self within days.
My second child is 20 months younger than the first. The first year of her life was stressful since her older sister was still quite young. But, I did know what to expect, got treatment for PPD before I really even had any symptoms, and managed that just fine. Now they are almost six, just turned 4 and we have a baby boy who is 5 months old. I am finally getting into the swing of this, and it is a really great job!
As everyone else pointed out, there are pros and cons. There are things I remember doing with my parents as a 5 & 6 year old that I just can't do with my oldest since there are 2 others tagging along. Yes, they do cost more, but we get hand-me-downs and careful planning doesn't make it so bad. They have eachother and derive benefits from that in ways that I couldn't offset. It all balances out. If you go for another, make it for you and your husband rather than for your daughter. The hard stuff gets easier because it won't be your first time. Good luck!