Just to say OP that you are going to get a biased view here purely because those who come to post are those most likely to be facing issues.
In real life, many women are very happy step parents, even, if not more, when having no children of her own.
I think it depends on a number of factors. How emotionally independent are you? re you looking for a nan to share everything or just about with or someone who will provide companionship but who you'd be happy to have interests outside of yours.
What is the relationship with the ex. Whether friendly or not, is he emotionally involved with issues, disagreement and these form a significant enough part of his life.
What kind of father is he? Do you have an image of what you consider a good father? Don't go by what he says, even says he does but what you actually witness. Often you won't get a clear picture until you actually move in together and witness it on a regular basis.
How are his kids? Expecting perfect kids is not going to go far, but where they are not perfect, are these imperfections you can live with?
To start with, you'll need to focus on what you see that falls under the no go areas. After that, it gets more complicated. It's often no black and white. I've met quite a few SMs who would say they wouldn't change things but who at times wondered what they've done and considered leaving. It's not just a case if SM or not, but SM to these children with this man....at a specific time. Sometimes it starts well and then gradually gets bad until there is no going back. Sometimes it starts not do good but gets better and ends up good and sometimes it's good times and not such good times, but gradually getting better overall.