By your questions & comments, I'm respectfully gathering you know very little about horses & sounds like you have a lot to learn before you're ready to look after one properly. Do you have some knowledgeable adults around that can help you out? Are your parents horse people? Is there a trainer/instructor close by that you can go to for lessons?
Especially as this is a 'first' experience, you really DO NOT want a young, uneducated baby, but find yourself a 'been there, done that' kind of a pony that you can ride. One that has reasonable ground manners, one that is good about having their feet & body handled, one that doesn't need to be taught everything you'll need it to know. Even if you don't start the horse yourself, you will still be training it every time you interact with it, and a teacher who doesn't know much is far more likely to share wrong lessons with her student... Best to start with an educated horse you can learn from
than one that needs teaching.
If you're wanting just a pony & donkey, you won't need/want much of a pasture. Horses are built to eat low grade roughage. Ponies more so. Donkeys even more so, especially a mini donk - I think, have evolved to thrive on some kind of twigs! Rich pasture is even more problematic to them as it can be for ponies & horses. A rocky bush block would be more ideal for them. Soaking hay before feeding is a good idea as it will leach out a lot of the sugars. Feeding it in some kind of 'slow feeder' will prevent them gorging on it.
They will need some kind of nutritional supp to balance their diet. I only know of one product (Khonke's brand) that is designed specifically for donkeys.
I'm curious why you want a donkey specifically? Is it that you really like donkeys & have some experience with them? I love them, but they're quite different 'people' to horses. As for 'bonding' them, if the horse/pony hasn't seen a donkey before, prepare for some funny business!
I've seen horses literally try to climb out of 7' yards to escape these terrifying aliens! Make sure there is a large, safe area, such as a large well fenced paddock, preferably that the horse already knows well(so not likely to run through fences), so the horse can stay at what she feels is a safe distance, then put the donkey in & allow the horse to take her time to get used to the donk without pressure.
You will also need to be prepared for worming, regular hoofcare(good farriers aren't half as common as... farriers & good farriers who know donkeys are rarer still), veterinary visits, yearly dentistry, etc.