wow, so much in your post that is familiar to me.
i met my DH when I was 37 and he was 50; had a baby with him when I was almost 40 and he was 53. We did not live together for the first six months of DD's life (my choice); we then moved in together and were married. DH has two teenagers who live with us full-time. He'd certainly not considered having another child, but I was not prepared to stay in the relationship if we didn't, and he came around quite quickly and was completely enthusiastic about the pregnancy and is devoted to the baby. I had same issues as you mention - financial insecurity, still wanting to travel, some differences of values and lifestyle, and on top of that, I am from overseas, so staying with him would mean staying away from my home country - while not staying with him would mean raising the baby far away from her father. His age, actually, was never much of an issue to me - he is very youthful in his mind, spirit and body, and is so active with his kids and now with DD - much more so than many men half his age.
For me, I was committed more to having a child than I was to him (at that point in the relationship, anyway). I was absolutely prepared for the eventuality that we might not stay together, and we discussed what that would be like. I was basically at a place in my life where I felt that I was ready to take a leap and have a child, and I guess that trumped all the rest. It s like purpleroses just said - I realised I and my child were going to be ok with either that best case scenario or that worst case scenario.
I think I have an unusual set-up in some ways, in that I have a bit more autonomy with DD than some married parents might have. I do take her on weekends away, and I have taken her for two lengthy visits to my home country, without DD and the SCs (we have also visited all together). I am the primary decision maker when it comes to DD; for instance, when it comes time for her to go to school, it is understood that I will make the choice of where she goes. Obviously DH has input and a sort of 'veto power' - and on a day-to-day basis, he is an extremely involved parent - but we agreed long before she was born that I would make the major decisions, and so far that seems to suit us fine. I guess it all reflects the way we started out and the fact that she and I lived separately at the start of her life. I think it is also a recognition of the fact that DH has primary care of his two older kids. Mostly though, it gives me a sort of security, knowing that I still have some independence and that she and I would be ok if we did end up on our own again.
Sorry that was so long! I will just end by saying that I have no regrets whatsoever about choosing to try for a baby when we did. She's my dream come true, and I can't imagine now having decided against it. She's also made my blended family much closer.
Good luck.