You are so young OP. You are in love and love conquers all, right?
The majority of posters are telling you to run for a reason. Marriage is difficult because there are 2 individuals trying to combine a life together with the pressure of jobs, money, kids, housing etc, and it's not easy. Add to it that we often get married without fully knowing our partner and what life is going to be like in a few years time. Someone I loved very much said to me years ago "often is not love that fails in a marriage, it's the everyday life that makes it fail"
You have 3 serious points to overcome
You are 24 and he's 36. The difference in living experience is enormous.
He's recently divorced. He has not experienced life as a single father with all the dramas of single parenting and looking for a wife's substitute to make it easier for him.
He has 3 children you'd be a stepmother for, with the huge responsibility they'll bring to your life.
If my daughter came to me in your situation, I'll be devastated. I'd try talking to her, explaining what she would take on, but ultimately I would accept her decision. A decision she'll make as a fully informed adult, which means she'd have to accept the consequences of her commitment.