well OP, either you are lying about how much your DP earns, or you are wrong that he's paying out 30% of his income, he clearly is paying out only a little above what he would have to pay on CSA calculators, so your original question is wrong.
Plus remember as htey are married, she's entitled to apply for spousal maintenance as well as child support. She is his wife and as such, has much more claim over his finances than his DP would.
However, you might want to look again at your plans, I'd delay until his full financial settlement is sorted, he might end up having to pay out more than he is now when they get divorced, have you considered that? This isn't the most he could be made to pay by a long way...
I really would not link my finances with a man who is still going through a divorce, and I would advise him to avoid the same with you until your divorce is finalised too (as I understand it, you have a non-working H you are divorcing, you are aware that he could apply for spousal maintenance from you? Especially as you previously have supported him financially. Obviously right now he's not and is being reasonable, but if he finds out about your affair and/or gets a good solicitor, you might find you have to pay out to keep him until he gets a job).
It's really not a good idea making plans for children until you know what both of you are able to bring to the table, and if once the excitment of the affair has ended if you can make a normal, dull, everyday relationship with your DP work.