Really glad to see you are feeling so much more positive & that you have a plan & support from your DH & other family members, stick to your guns, this is a really good start.
I also second reading the other links & you & your DH familiarising yourselves with the "Toxic Parent Script" it is actually quite an eye opener & will help your DH cope better & not crumble WHEN his DM follows it to a tee - & she will -
my own DM, despite really having several serious health issues, still conjured up imaginary heart attacks & strokes to make people toe her line - in her case, as a diabetic, she would self harm by eating sugar rich foods & ending up in hospital - once there, the woman who was too ill to ever ring me, couldn't see to use her mobile phone etc, would suddenly be able to use that phone to ring me literally gloating that she was in hospital & had had a heart attack - on one occasion she had a heart attack AND a stroke - she had neither, the hospital had taken her in to monitor & sort out her very high sugar levels - very high because she stuffed herself full of sugar - on purpose
So preparing yourselves for this will only serve to strengthen your case with your DH
The other important thing that I can't stress enough if she's to be in you DDs life, is that you bring your DD up to understand that YOU are her parent, therefore YOU have the last say - do not be afraid to undermine your MIL to your DD, your DD needs to know that your MIL doesn't call the shots, you & DH do.
I say that as having know a lady who was too conditioned & afraid herself to adequately stand up to her bully of a DF & his side kick DM - even though she moved her family away, they took her to court for access to her son - ended up with DGP lavishing the boy with expensive gifts, court order to have him stay over during holiday & letting him do anything he wanted, all against the DMs will, mobile phone to young, dangerous sports, even a hand gun & shooting club at 8 years old - all designed to win favour & take control over the DS - my friend went through hell.
My own DM followed a similar pattern with my DD - parcels full of gifts every month, often stuff she knew i would never agree too - even a bra & pants set - at 5 years old 
boxes & drawers full of gifts waiting every time we visited - even bags & bags of sweets DD can't eat because of intolerance - given with comments such as - "I know you would like to eat these really, your mum is really mean not to let you, but you know I love you enough to buy them for you"
A chance conversation with my DD threw up something we as adults can too easily forget & my friend whose DS I mention above was shocked because it suddenly dawned her that it was the key to her own problems with her DS taking her DPs parenting over her own & had she realised sooner, she might have been able to save the situation.
My DD believed that because I was her mum & Nana was my mum,therefore in DDs words " Nana is in charge of us all" - I had the opportunity to explain that isn't how it works & as a result my own DD always knew I had the last say - NOT Nana
Something I realise has made things a whole lot easier than it could have been