HI there, I am not sure I can offer any helpful advice, but wanted to give you a reply, so like chelen i can only tell you of my experience.
I am married now; DH has two kids and we have a daughter together who is 19 months.
In my case, DD was planned, but staying together was not a given - I was prepared to raise DD on my own, living separately. I was unwilling to live with or marry (now) DH unless there were radical changes in his home and in regards to his parenting. He took that on board and has, in fact, made huge changes. We moved in together as a family when DD was six months old and got married not long after. It was all a bit unconventional and backwards, I suppose, but it was the right way for us.
Things worked out well as far as the baby - she has been great for us as a family; the kids love her to bits and she definitely has made us all more connected.
DH and I do have problems, and at times I have considered leaving the relationship, but DD is a bright spot in all of that, and like chelan says, again, the kids do hold us together in a way...if it were just him and I, we might not have stuck it through some of the worst times -- but then, if it were just he and I, we wouldn't have actually had the same worst times, as our big issues really are all about blended family... [shrug]
I understand what chelan means about not having the energy to be a step and have another child. I can't do both, so will probably not have a second child myself. In another situation, I probably would. So I feel some resentment about that, maybe.
If I were to offer any advice at all - I guess it would be to ask yourself how things will work out in each possible scenario. For me, I thought (and discussed with my DP of course!) about how it would work (logistically, emotionally, financially) if we stayed together, and if we didn't.
Good luck.