It's very exciting - and also a bit scary... we've both been married before but (he in particular) has been badly burnt financially when the marriage ended. We'd like to make sure all legal and money conflicts have been covered before he actually moves in - so what do we need to consider?
He'll be moving into my, mortgage-free house with DD (4yo) and 2 cats. His two children (12yo DS and 6yo DD) will stay with us every second weekend. The girls are happy to share a room, and DS will have his own.
How do we sort the finances? I'm currently on IS and CTC, which ends when he moves in, and have started a business which is small but has potential of about £1k a month. XH pays me child maintenance of £300 a month. CHB is £87 per month.
DP has an 8-5 job paying around £2200 after tax. Money is tight for him, as he's currently taking about £100 out of his savings each month after expenses, which is not a sustainable situation. He pays child maintenance to his XW of around £450 pm. He owns his own flat, which has a monthly mortgage of £800. He will rent this out, bringing in around £1k per month, which after tax, ground rent and XW's cut leaves him around £450.
Bills on my house will be higher than at DPs flat (bigger place, more expensive area) so he'll inevitably have to spend more living with me. Food and going out should be much cheaper, as we can just stay in and "make our own entertainment"... 
However we've crunched some numbers and it looks do-able as long as the flat is occupied and his salary stays the same. How do we sort out bills? Shall I continue to pay them, and charge him rent for being here? Or do we open a joint account and each pay in a share? If so, what should each pay?
Also, how do we do this legally? Do we need a formal agreement about who pays for what, who does what around the house, what happens if we buy stuff together etc?
Sorry this is a bit long - we're just trying to be as sensible as we can in the beginning, so that we can have a stress-free life together... Thanks!