I was in the same boat as you. I was happy with my one son, had worked very hard for my career and was concerned about money, life, etc. I had my first son when I was 34 and my husband and I thought we would only have one. I was away for a few days and when I came home, my husband (who was VERY consistent in his desire for only 1 child) said to me; "we NEED to have another baby, because (our son) needs a sibling experience." Weirdly enough, I was already, by accident, pregnant. I was 37.
I now have a healthy almost 3 year old and a seven year old. It was a healthy pregnancy, but a little harder than the first. I think the older you get (and it's not like you are old) it is harder physically to carry the weight and stress on your body, but definitely not terrible. Juggling 2 is definitely harder, but I was glad to have another chance. My second one is VASTLY different than the first, so it's almost like figuring out parenting again.
As far as money is concerned, the only thing that is a bummer is having to pay for daycare (we nanny share with another person at our house) when we were almost done with that. But, it's only 4 years, and the person you produce will be around for much longer than that.
It's funny, now I wish I had started earlier, because I'd love to have another, but I am 40 now and facing the same questions you are, but the health issue is more intense. I would not worry about having a baby at 36 or 37 or even 38, as long as you have proper prenatal care.
To be terribly honest, the thing that is the hardest, for me, is managing time. I work full time and have one child in school and one that is home. All of my energy goes to kids, homework, keeping them organized, keeping the house semi-straight and keeping healthy food in the house. Luckily, my relationship with my husband is very much a partnership, because we have had little time for "us".
That's all I got! Good luck with your decision.