Lots of handling of mum and bub, let them come to you before you start trying to be with them too much or there is a chance they will turn away from you and not be interested. I've had foals before and we found that when they were approached themselves rather than playing with mum and bub being incidental the foals were more attentive and affectionate than those we had tried to focus on more (before we realised we were pushing them away). This isn't by any means definitive but just something we found worked.

Good for you in removing these horses from a life of misery. I'm assuming you have a good vet working with you but it doesn't sound as though they have been scanned yet? what is you bad feeling about one of them - do you mean it is not moving or...? When you say 'unusually aggressive' behaviour that makes you think they are in foal what are they doing? What environment do you have them in at the moment, I know they were starving so I'm guessing you have been feeding them well, could that have anything to do with it at all do you think?

As for a 'foal kit' most people I know just make sure they have a nice quiet, well bedded and spacious box and leave Mum to do the work - however, one of my clients has a golden rule "If the foal is not on the floor within 15/20 minutes max. get the vet in immediately". After the foal is born make sure all the after birth is passed and intact. There are far more experienced people than me on here about foal management so I'm sure you will get lots of advice. Have a think about imprinting - it does work. Turn the foal back out with Mum (weather permitting) within a day or so; foal needs to be used to being handled but not too much. Get them used to a halter, being lead, and feet up and trimmed, otherwise, leave them alone I think :-)

Hi!
Congratulations on rescuing these horses!!!!
Coming back to your "foal kit"-question, I'd suggest you inform yourself about what,when and how, for example by reading "Mare, Foal & Stallion Care" Schweizer/Cable/Squires, published by the Horse, health care library. This book or any other good book can prepare for all eventual cases etc. Don't forget to ask your vet to deworm and vaccinate the mares in time before they're due. At least have a bucket with plastic bags ready to keep the membranes in, so you ( better: the vet) can check them for being complete. This way you'll know when to call for help in case part of the membranes didn't come out timely.
Keep an iodine-solution at hand for the navelstump. Ask your vet about it!
Sometimes you need to pump some colostrum from the mare, to make sure the foal gets the essential antibodies in time. For this reason it's good to have a babybottle, so you'll be able to give it to the foal, should it not be able to suck well.
It good to know about foal's heartbeat at birth and after, so you'll be able to see if all's going well. Be sure you feel competent assisting at birth, if not, don't be shy to call for help in time. If you feel you can do "imprinting", sounds great, but be sure you're doing it right. this precious moment in a foal's life cannot be repeated.
From what you write above I gather there's some time before the mares give birth, so use that time to be well prepared.
Let us know how things go!
Good luck,
Miriam