To start off with you have to pay child support at 20 per cent of your net income. To some extent this depends on how often you have the kids. You will find a useful calculator on the CSA website. There is little choice about this.
Anyway let's start with £660. It is likely to be in that region and there is little choice about it.
As regards the house, can you please let me know the likely value, the amount you owe on the mortgage and the monthly repayments.
Does your wife work ? Would it be reasonable to expect her to ? How old are the kids ?
You then need to consider your wife's likely benefit entitlement, particularly tax credits and child benefit. This must be counted as part of her income and reduces her need for SM.
The chances are that she will get the right to live in the house at least while the children are still dependent. Therefore you will need accommodation of your own and you must be left with enough to rent a house in your own right. Ideally this should be large enough to allow staying access.
I think that's enough for the moment. I have more to say ; but Please answer these questions first. Your wife's demands are totally over the top.