There are a couple of websites that will give you a rough guide as to what benefits a single parent could be entitled to but remember that maintenance is not means tested against benefits so really its a pointless exercise in trying to find out. Even if you did it is not right that you should then sit with those figures and decide for yourself how much his ex needs to live on and support their child. That is her own business. It doesn't affect the maintenance at all.
I think the most important issue you should be considering, far more important than financial planning for any future children is trying to help your partner's daughter settle down into her new family structure. Yes, millions of children's parents are separated but that doesn't make it any easier to deal with when it happens to them and it affects a lot of them for the rest of their lives, affecting their schoolwork, their friendships, future relationships and potential future prospects. You seem to underestimate the rolling impact it has on both parents, the children and any new partners trying to build a relationship. To rush in at this point and actually plan a baby when the dust hasn't even settled yet with your partner's break up (and believe me 6 months in a marriage breakdown with children involved is nothing compared to the years these things can rumble on for) shows a lot of niavety and lack of understanding about how things are for others involved in the situation.
I say this as a stepmum to 2 boys for 8 years who STILL struggle with their parents living apart and whose dad helps out financially over the top of legal requirements without question. Yes the early days were fraught with DH and his EX both trying to reset their new boundaries but thankfully those days are gone now (and is a normal part of a marriage breakdown and they have to be allowed to go about it without interference) and we are all at a more civil point in our lives. It has taken a long time to get here, many tears, arguments and solicitors letters.
The maintenance payable is set out in the CSA calculator, is payable by your partner and is deemed HIS legal responsibility, not yours too. Whilst you may feel it's coming from your pocket too he still has to pay that whether he is with you or not. You need to mentally separate it. When we do our monthly finances the maintenance payment is up there with our mortgage and council tax - its a priority. The extra payments for their children are made as and when DH's EX approaches HIM to request the assistance. He makes the final decision but always lets me know first as a matter of courtesy that money will be leaving our bank account.
I really feel you need to take a step back and ask if you are ready for a life of being a stepmother, a partner to a man with an EXP and child and for a lifelong financial obligation to somebody else's child (we all know money doesn't stop being spent when our children turn 18!) It can really mess with your head if you are not in the right place mentally to start with and I have seen a few relationships breakdown after 2 or 3 years before children have even come along in the new family...