Assuming you have informed him in a minimalist email that you have now told your dcs in an age-appropriate way (with no mention of the d-i-v-o-r-c-e word which is way above their comprehension) that their df will be living elsewhere and you fired that particular salvo off last night, we can now turn our attention to his mention of mediation 'to save on fees'.
I would hazard a guess that he's spoken to a divorced friend/colleague or has spent time searching 'divorce adultery' online and is clutching at straws has come to the conclusion that he'll be able to bamboozle you in mediation sessions while saving on solicitors' fees.
You're under no obligation to consider mediation at this stage as attendance at MIAMs (mediation and assessment meetings) only becomes compulsory when warring divorcing couples cannot agree on the finances and/or the childcare arrangements and intend to seek court orders to resolve their disputes.
However, it should be noted that spouses who have been subjected to domestic violence/abuse are exempt from this recent rule and there is no requirement for divorcing couples to actually resolve their differences through multiple sessions of mediation.
Fwiw, mediators are not allowed to dispense legal advice and, if you would feel intimidated by sitting in the same room as your h, you can ask for shuttle mediation whereby the two parties sit in separate rooms while the mediator 'shuttles' between them.
I would suggest you send a further email tomorrow evening or Saturday morning saying that 'With reference to your suggestion of mediation, I understand that this will only be necessary if our respective solicitors are unable to reach an accord with regard to financial and childcare arrangements. As I will be asking only for that to which I am legally entitled I do not anticipate that mediation will be required and, in any event, I will be led by my solicitor's advice in this matter".
That should cheer him up for the weekend make it clear that he won't be dictating terms to you. Unless you are able to come across as being coldly dispassionate, I would suggest you resist the temptation to put my words into yours because he'll be looking for a chink in your armour and will seize on any turn of phrase you may use that suggests weakness or wavering.
As for what you're legally entitled to, given your circumstances as stated here and the ages of your dc I would suggest 80% of the equity of the marital home, 50% of all other property/investments/savings/etc, child maintenance, spousal support, and 50% of his pension fund.
I suspect that during all of the hours he's spent fantasising about his wonderful new life with the ow, the dick-led idiot saw himself as having more or less the same disposable income and assets that he currently enjoys... stupid twunt!
I'm wondering how long it will take him to reach the conclusion that he's made a terrible mistake, that it's you he truly loves, and he starts begging for another chance.