How to go NC, the practicalities?
Since they live 10 mins away it's extra difficult. I take it there is no chance of moving (it's a very tall order admittedly). If there is any chance - take it. Seriously, it will make things SO much easier.
Regarding the debt, set up a standing order for it. Is there any chance of askign a friend or the bank or micro credit union for a loan so that you can pay her off in one? that will help a great deal. If not, simply keep paying it back by electronic transfer so there is a record of it.
For the rest of it, you have to plan how to handle it. It's hard being in touch with them, and it's going to be hard changing things so you're not in touch with them. Ever so restful afterwards though!
The best way is to do it quietly. Stop beign available. Make excuses for not popping round or not being available to meet. Not every time, start off with sometimes being unavailable and then gradually increase your unavailability. Answer the texts, but leave a period of time before answering. Sometimes make it the next day. Start not answering at all. If they pop round let them in now and then but start saying "ok i need to do XXX now, I'll be in contact, see you soon". If you have to, say that you're going out (and go; it's an arse but it's all part of the long term plan and won't be forever).
At some point they will probably get nasty. You need to plan how to deal with that. I'd suggest calmly under all circumstances. Think what buttons of yours they're likely to press and plan how you will answer them and handle it, in a calm way. If they start shouting or getting abusive, you can tell them that it's not okay to speak to you like that and they have to leave or you will call the police. Of coruse, you do have to back up what you say. But it will show them you mean it - because you do.
Most of all, keep them well away from your daughter. In age - appropriate terms tell her why it's better that you don't see her aunt and grandmother. And if you really want to make a clean break, next time they say something rude about her tell them it's not ok and you don't want to hear it any more. that should really set make them angry and then you can walk away (but keep paying the debt back; but there's no need to contact them about it).
Absolute worst case you can send them a letter and tell them you want no more contact and that any more will be considered harassment, but hopefully that will not be necessary.
Sometimes though, people actually back off once they realise the person they are walking all over is now standing up to them. If they do back off and start treating you and your daughter better, would you want limited contact? or would you want to stay well away? Just something to think about.