I would definitely go for 75% of the house, because if the OW already has a house, he can, and I bet he will, when the dust settles, move in there. He will have a roof over his head, come what may.
In the meantime, he's playing the 'poor me, I have to stay at my father's house on his sofa' act. Don't fall for it and let your solicitor know absolutely everything.
I think you really must get a top notch solicitor and take him to the cleaners. You absolutely must speak to a solicitor before you start thinking about remortgaging.
The IFA can't sort out the best mortgage deal for you, until you know how the house will be split and it's current valuation. I would be very seriously inclined to get my own valuations and pick the lowest three. I bet that's why your IFA is floundering and you can't get a mortgage at the moment. Basically there are too many unanswered questions that need to be settled before any IFA can sort anything, except the value of your house. Even then, I would compare him with another two IFA's, to see what figures they come up with.
Having worked for a relocation company, when house prices were relatively more stable than they are now, it was absolutely amazing to see just how much of a difference there can be in different estate agent valuations. We used to use three, but when there was a difference of more than 5% between the lowest of the highest of the valuations, we'd get a fourth valuation and keep getting valuations until there was less than 5% difference between the highest and lowest. If you can show him and your solicitor a couple of the lowest valuations, then you'd have to potentially pay less to buy him out.
The OW's house can't be too tiny with three children, that's for sure! I wouldn't mind betting that when he thinks he's going to get his 'new fat lump', he and the OW have romantic notions of buying a bigger house. You ex sounds extremely cunning and knaving bastard to me. He's definitely playing you, I'm afraid to say. Don't fall for it whatsoever.
Personally, I think it's very dodgy talking about anything financial or anything else with you. He maybe recording your conversations. I would repeatedly tell hm to liaise through you solicitor only and that you refuse to discuss absolutely anything with him whatsoever. Play the broken record player and keep telling him that. Don't be misled and just keep telling him to speak to your solicitor. Even when he phone's, just keep reiterating to him to speak to your solicitor. Don't get drawn into conversation with him whatsoever. He's already played dirty, so you have every reason to play dirty back. I would think that you're are far more likely to get a mortgage to buy him out if you went for 75% (or more if you can get it).
Personally I, (and I expect other users on this post), would like to have him physically pinned to a dartboard (plus a few other men who've played dirty!), so we could all have a go at getting more than a few 'balls' eyes, with some nice rusty darts. It's no less than he deserves for doing this to you and your DC.
It would be in utterly unfair if you allowed him 50/50, because then you'd be left struggling financially, whilst he goes swanning off into his sunset. Or that's what he thinks. In fact he'll be taking on the financial cost of the OW and her three kids too. He'll soon realise the grass wasn't so green on the other side of the fence and he'll attempt to come crawling back. The kids may want you to have him back, but gently remind them just how much he's hurt you all. Tell them that you just cannot trust him not to do it again.
Once a cheat, always a cheat. That's why the divorce rate for second marriages are even higher than first (which is around a staggering 48%!).
Apart from getting a solicitor, speak to the CAB; a friend of mine found them invaluable when her marriage broke down. They advised her on all the benefits she was entitled too, which has been a godsend to her.
We're all rooting for you, wanting nothing but the best for you. It's obvious too, that all the OP on this thread genuinely care and are feeling somewhat angry to what your bastard ex has done to you and your DC.
Your children may go through all sorts of emotions, including some directed at you, because they are misguided and almost certainly given some sob story from your ex. Especially when the inevitable happens and all goes tits up with the OW. Your ex is in Cuckoo land, but that is most definitely not your problem, whatsoever.
Go for everything you can, if not for you, then for the sake of your DC. It's definitely not their fault that your ex has been a total arsehole.
Remember Relate to one to one counselling too, if you feel your head has become totally overwhelmed by all this.
I'd be very inclined too, to have a chat with your GP regarding the mental health of your children. There's no stigma attached to it now. It's better to get them the help that they're going to really need from an outside, but safe and professional psychologist. Because they're under 18, when they apply for a job in the future, they don't have to declare it to any potential employer. The GP can get a referral to CAMHS (Children and Mental Health Services) and urgently ifnhe feels it's warranted. I did that for my DD as she does pent things up and she just wasn't coping. She was falling apart in front of me and there was nothing I could say or do. I now wish to the moon and back that I'd done it for my DS too, as he now has anger issues. Most recently it involved the police and he came very close to being arrested and charged. It was beyond words as to how painful it was for me. At the time I couldn't get through to my DD at all. Even her friends were worried about her as she clammed up and became a selective mute. I was seriously worried. I thought my DS was ok, but in hindsight, I now realise he wasn't. Children can sometimes feel that a failure of a marriage is their fault, but it's not. In the end, through a counsellor with whom DD felt comfortable with (she couldn't cope with the first, who was a fairly young man) she absolutely blossomed 100%. She was given cognitive behavioural therapy. I think that if I hadn't have done that, she'd have not got through uni and got such an excellent job. I know that she still uses many of the strategies that she was taught.
In hindsight, looking back, I could physically kick myself for not picking up on my DS's anger issues. I know the cause wasn't my fault and totally beyond my control, but it literally eats me up, knowing that he's got these anger issues, he's had several failed relationships that have crashed and burned spectacularly and he's not coped whatsoever. The guilt of not picking up on the clues at the time is immense. The fact that he used me as a punch bag several times,as well as the walls and a door in this house, bear testament to his anger. As his mother, I feel I've failed him completely. The guilt is literally eating me up and tearing me apart inside. I truly really fear for DS's future; whether he'll get through uni, having failed one year, whether he'll cope to the end, whether he'll get a good job and how the hell he's going to cope with any further relationships. I've asked him to just play the files and enjoy life for the time being. I've
told him he's far too young to be involved in a long term relationship. I just so wish, so very deeply, that I got him help too. OP, even if your DC only end up needing one session, at least you won't be in the mess that I'm in right now. And you won't be blaming yourself if one off your DC do go off the rails, unlike myself. Guilt is a heavy burden to carry. I've sought help and in one session, I've got my answers. I think I just need to write them down and read them often, to remind me, it's not 'my' fault.