i'd be more concerned about you feeling the need to justify why you only want one! you've obviously thought alot about it and I wonder if you are trying to convince yourself or others?
I always say to my friends who don't have kids yet... you've got to really want them cos they are hard work! if you don't want another one, then it's really no one else's business.

If 1 is all you want then be happy within yourself about it and go tell anyone who tries to make you justify your decision or feel like you should be pumping them out to go hang. Or just inform them "i'm not going to have a second child just to make other people happy". lol they'll get the hint quick enough!

Enjoy watching your little one grow! and enjoy being in the enviable position of being able to really lavish your attention on her... I had two close together and sometimes wish I had more one-on-one time with both of them...