Hi & welcome Olivia,
Sounds like you've got lots of homework to do to get ready for your pony! Enjoy the journey!
Depends on pony's build, etc, and if you want her to get your confidence up, you're not going to be doing much more than walking & trotting for the moment anyway, but even at your size, do keep in mind you could be a bit big for her, so just go easy.
1. What gear will I need for her? Any recommendation on which brushes, what blankets she'll need (I live in Keith, SA), lead ropes, halters, bridles, saddle etc.
2. Saddle? What size saddle would I need for a 12hh pony
A normal medium hard brush will probably do. Depends, but most horses don't need blankets, so long as they have some sort of shelter, esp from wind, and they grow a winter coat. If she's not used to a blanket, and while the foal is at foot, best avoided for now at least I reckon, but if one's deemed necessary, the lightest one you can get away with, used on the coldest days should be fine. I personally like rope halters, some don't. Don't leave halters on horses when loose. Long, yacht braid ropes are best IMO. at least 10-12'. A snaffle bridle is probably fine, assuming she's been trained. If not, a halter/bitless bridle is my choice to start a horse in. Assuming she is already well trained under saddle tho, if you've bought her because you're nervous?
You need to find a saddle that fits her first & foremost. There's no one-size-fits-all-12.2hh & they're all different shapes. You will find some good saddle fitting info on youtube & google. SpinalVet : Spinal Conditions and Treatment for Horses and Dogs
is one place with some good basic saddle fitting stuff. Balance International also has some good info on their site on saddle fitting & problems. Once you know which saddles suit your horse, then you can consider seat size for yourself. *Which may cut down options, as for eg. a Western saddle that fits you may be too long for the pony - an English tree is shorter in relation to seat size.
3. Feed? How much feed will she need per day/week and what kind of feed is best?
Horses need around 2% bodyweight daily in forage(hay or grass). They need free access, or little & often feeds, not good to go hungry or be fed rich/large meals. If they require more energy, due to weight loss, hard exercise, etc,(or lactation) it's best to keep to low starch ingredients as supplementary feed, such as beet pulp, alfalfa, rice hulls, etc. In addition to that, well balanced nutrition needs to be considered, but it depends what she's getting in her diet as to what she may need. FeedXL.com is one great resource for working that out. Growing babies & lactating also pulls a lot of nutrients from a mare, and I'd hope she's being fed appropriately now. If so, I'd continue with that for a while after baby is weaned.