What is the plan for him looking after DS for the next few months? Before you change the locks you need to sort that out so that it's not happening at your home, or you'll just end up having to get him a set cut.
I would also be a bit wary of letting him know you are still pissed off about it and mentioning divorce until you have spoken to a solicitor or two and see how best to proceed. Antagonising him and having him decide he's going to move back in isn't going to be in your best interest.
You don't need to have a list of things to discuss with the sol's - they've done this a lot, a good one will ask you the right questions. What you do need is a solicitor who wants to go for the jugular and who will fight your corner, not one that is apathetic or says you wont get much etc. It's best to go through a few at this stage than end up with a crap one.
Just keep talking to DD - she's old enough to understand why you are upset now (he's not trying to make it work, he's being selfish - rather than the affair itself). It will do her no harm at all to know the broad outline of what you are going through. She's not stupid and it's more scary not knowing. Just also keep reminding her that she only told you about the texts, she didn't do anything wrong and if she hadn't you would have found out in time anyway and been more hurt because it would have been going on even longer.
Mtce is based on a % of his earnings. I'm not sure if it's 15% or 20% for two kids - but I'm sure a quick google would tell you. However, the fact that he's going to have them 50% of the time would be taken into consideration - so I'm not sure you'd get much if anything - from him. You'd still get the benefits I think.
Would you be able to stay in your home on your wage alone?