I haven't seen anyone else post about choosing to stay with one child, so here goes.
I had my first child, a daughter, when I was 41. It was a happy surprise, with a great pregnancy. Delivery ending up going to an emergency C-section because of complications, but both the baby and I turned out fine. She's currently 18 months old.
I, too, mulled the possibility of going for another child. I felt that there was another soul knocking on my heart. But, my husband definitely isn't up to it, at this time in our lives. And, I don't really have several years to see if he becomes more comfortable with the idea. Plus, I like being able to focus on parenting my one child (which I'm finding to be a handful -- rewarding, worth it, but super-tiring!), while also trying to balance husband, work and personal needs.
As she's getting older and is needing more stimulation, guidance, etc., I've thought twice about my wonderings about another baby. As it is, I'm looking at SAH mothering until she's maybe five, with work stuff being part-time (and, I love my work). If I had another baby, I'm looking at ten years of parenting small children. By the time they'd become more independent, I'd be in my early to mid-50s. That prospect doesn't appeal to me so much...
I also felt "lucky" to have had a perfectly healthy baby. If I gave birth again at 43 or 45, would I still be so lucky? If I had a special needs child, would I be up to it? For me, I highly doubt that I'd abort, unless my life was at stake. Ultimately, I think I'd rally to the challenges, but I think it would overstretch my DH; and, I'd feel bad about pushing the additional baby idea.
There's no guarantee that bearing her a sibling will ensure a close sibling relationship. A good chance, yes, but not a certainty. Don't do her any favors! Ha ha. If you both feel a yearning for additional children, so be it. But, don't do it for your DD. She's going to end up having her own relationship with any siblings, and who knows? she may really resent sharing you two!
To create a nurturing cirle around my DD, we have close friends who have children in the same age range as our DD. We have dinner with them every Friday, and often do other activities together during the week. I also make sure that we have activities to go to with other babies/children four or five times a week. Our family are all plane rides away, so we travel about four times per year to visit everyone, and/or they come visit us. However, that's how I grew up, so I'm used to it. And, we use our Webcam a lot...
Ultimately, I agree with the poster who said to go with your gut; especially, if your husband is feeling a "pull," too, for another child. But, if you feel it's ultimately not in the cards for your family, that's okay. There's no rulebook that says you're only a good parent if you have more than one child. :)