Take everything slowly. You've mostly done this so far but let a child get used to one change at a time (meet, wait, meet the kids, wait, spend more time with them, wait, get married, wait, move in, wait, and only then start thinking about more kids).
Make sure your partner is the absolute love of your life and you can't imagine being with anyone else. That has kept me going through the hard times. Otherwise, find someone else without kids.
Don't be naive about the great relationship with the ex.Make sure there are court mandated access arrangements and if they've been married a financial clean break order, a decree nisi etc. My DH's great relationship with his ex rapidly deteriorated after we got married. It has been a nightmare and we have been through a lengthy and expensive court case to maintain decent access to his kids.
Be prepared for the sacrifice. Financially you have extra dependents, your house needs to squeeze more people in, you won't get quiet family newborn time with just your partner, your free time will rarely be spent between just the two of you. You need to make a lot of significant sacrifices to make it work and to make your SD feel included. Be prepared for this.
Your SD may bring a completely different set of values into your home -- her mum may well have different attitudes to social media, language, manners, cleanliness, school, bed times etc. Building your own household values as a couple and then a family is only ever partially possible and very hard work. Will you be able to cope with this?
Discuss your responsibilities and rights. If you are going to be expected to do school pick up, is your partner also comfortable with you telling her off? If you are going to be buying clothes and making tea, is your partner comfortable with you also having input into expectations around behavior?
I don't regret being a step parent -- I love my partner, our home and our family. My relationship with my step kids is good and my husband's relationship with them has continued to be wonderful. But it has been so much more painful and complicated and stressful than I anticipated. It has been the only real source of tension between me and my husband and there have been brief but real moments where I considered walking away.