Thought I would jump in here and add my thoughts.
He will throw plenty of emotional hooks at you and refuse to accept your decision. This is a defence mechanism because he doesn't want to accept the fact that you have decided to move on. Blaming you for the state of the marriage, his problems, the harm to be done to the kids, your financial future are all common.
Ruining the childrens lives is a common myth used as a tactic to make you give up on ending the marriage. See link below
wendyparis.com/yes-you-can-raise-happy-children-after-divorce/
research shows that kids need to have a good relationship with their parents which is irrelevant of marriage or divorce. In fact when you are together the kids will be assuming that this is how couples treat each other so any arguing, silences, awkwardness, bickering, snapping leads to the kids thinking that this is how couples behave.
You have spent a long time coming to this decision, don't expect him to accept it, he won't and that is no reason to stay. You health and happiness is important and you must do whatever you need to do to achieve this.
Financially things will slot into place but right now it is important to get that weight lifted from your shoulders. Take legal advice with a free half hour consultation with a solicitor.
With regards to when to tell the kids, play that by ear. If you tell them before Xmas it could put a sadness over Xmas, but you will have the holidays to discuss with kids. Tell them after Xmas and the kids could see a happy Xmas as a lie and go back to school. There isn't a good time. You could start by saying to the kids you think you both need a break as you are not happy rather than launching into a divorce conversation.
Good luck.