Agree with this. Am going through a divorce at the moment, and have been shocked how quickly things turned sour and difficult once we got down to the nuts and bolts of the finances. I've been - very much with his consent - a SAHM for years and the children all still live with me, but he's tried to really screw me over financially. He like your H runs his own business and I found out he'd been seeing other women.
A good solicitor has saved my bacon, and is guiding me through it all. It's SO important you think long term , and protect yours and your children's futures. 50/50 may well not be in your best interests as you will be 'on the back foot' after being a SAHM & it sounds like you will be primary carer going forward.
Don't worry about finding his financial paperwork etc, as part of the divorce he will be required to give full financial disclosure. Again, a good solicitor will guide you through.
I filed for divorce online - cost iirc c £600 - then had a 45 min appointment with a family solicitor. I've stuck with her, and she has been dealing well with ex's shenanigans. It's a real shock when you suddenly find yourself faced with someone you really don't know, despite living with for years. As others have said, he's not your friend - don't be afraid to be called selfish, unreasonable, heartless etc by him now. Concentrate on you & the dc.