I agree with everyone saying you must see a solicitor ASAP.
In my experience, you have about 6 weeks of them feeling somewhat contrite over what they've done, and they'll offer all sorts at this stage to get out "clean" without a raging battle - but then it wears off and they start to think "hang on, why SHOULD I give her XYZ, that's not reasonable!" Especially if there is someone else (like there usually is, the times there aren't are many many fewer!) because they're quite often in his ear saying "but why would you give her all of that? that's too much..."
Mine walked out on my a few months before our wedding. Claiming there was "no offer of a relationship" -ahahah, yet he moved straight in with her.
I told him HE had to tell his friends that the wedding was off, and why - do you think he bothered to? Did he fuck.
So after about 6 weeks of him being gone, I had to field an extremely awkward call from one of his best friends, wondering what had happened to his wedding invitation?
Do NOT rely on him telling anyone - he won't unless it suits him to, which it won't in most cases. I'm glad you've told some friends - tell more, tell family.
My mum gave me one sound piece of advice - never expect the best from him. Expect the worst - absolutely do this - and you can then never be shocked or disappointed, but you could be pleasantly surprised. Otherwise you'll live in a state of permanent shock and upset every time he goes another rung lower on the decency scale (and again, MOST of them do - the odd exception is not the bar to set for yourself or him)
Do not take this offer without seeing a solicitor. Sign nothing, agree to nothing - get legal advice and get it tied up tightly. ESPECIALLY if he suggests that you can manage it all without involving a solicitor - that means he's probably pulling a fast one and he knows it.
I'm still very sorry that you're in this situation - but many many of us have been here before and have the battle scars to prove it. Good luck and yes, rant /vent away on here whenever you need to!