More importantly, OP, focus on compatibility and maturity. I'm on my 2nd marriage (and last, God willing), and she's a good bit younger. (I still meet the xkcd criteria, though.) But she's very mature, and we're very compatible.
I also recommend a book: Saving Your Second Marriage Before It Starts
Good luck!
I'm glad that you are feeling things so positively!
I personally started wrapping my head around the idea of being a stepmom by reading books, and by browsing this subreddit. It's a thing that comes with a lot of emotions that people in regular relationships don't have to deal with, and it's something that's often not talked about, so someone who is brand new to the idea of being a stepparent might not even know the kinds of feelings they're in for if they pick this path for their future.
So, my book recommendations are "Saving your Marriage Before It Starts." This one is actually not about stepparenting but it's about relationships and I think everyone should read it. If you would consider yours or his past relationships serious ones, consider reading "Saving Your Second Marriage Before It Starts." This is the one I read and it has chapters on stepparenting which are helpful. https://www.amazon.com/Saving-Second-Marriage-Before-Starts/dp/0310346304
The book "A Career Girl's Guide to Becoming a Stepmom" was hugely helpful for me, because of how into my career I was at the time. Framing up Stepparenting as a job with goals and pitfalls was helpful, since I wasn't used to living a parenting life so a parenting book didn't seem right. https://www.amazon.com/Career-Girls-Guide-Becoming-Stepmom/dp/0060846836
That book in particular does a good job of pointing out problems that weren't even on my radar. "What happens if you earn more than him because he is paying a lot of child support, and he asks for you to pay what you feel is more than your fair share for housing/food/utilities?" Or things like asking to see the official court ordered custody plan.
Also just showing how much you sacrifice when you are with someone with a kid. You likely can't move anywhere you want for a career or if your extended family moves somewhere else. You don't get a child-free life, but you also don't get to be a true parent. Your schedule is often dictated by things beyond your control. You don't get to become parents together. These are things you have to mourn when you choose this life because if you don't confront the feelings, they turn into resentments.
YOU are picking this life. The child did not pick this life, and your partner didn't plan on being a single parent. But YOU are the only one in the equation making the decision to join this family, which means you have more responsibility over your own self and feelings, to know if you are capable of taking on such a complicated life. Only you can know "can I deal with these selfish thoughts of wanting it to be us with our child and not this other child?" If you can't handle it, then it's not the right relationship. Unfortunately, the only thing that will help you learn about yourself is time, and during that time you'll be getting more attached, which means that if it DOES become too hard, a lot of times stepparents feel trapped with someone they love in a situation they didn't understand would be so freaking hard.
So yeah. Keep browsing this subreddit, read some books, find some books of your own that seem like they might be your style, and seriously just go into it with the most open mind of "I don't know what I'm in for so I'm going to try to make the right choice now, before I get so attached that the decision feels made for me."