As far as the miniature donkey is concerned.
Mini donkeys fit most large miniature horse equipment; including blankets and halters. Depending on the quality, the prices for them can fluxuate quite a bit. For example, we bought a show-quality 37" Miniature Silician Jack for $600 last year, but were able to get a second show-quality 35" Jack for $150, simply because he lacked the 'spunk' needed for the ring. It really pays to do your research, and find a breeder of good-often handled-well build miniature donkeys, then let them know that you're looking for a pasture buddy for your horse. Chances are they'll contact you with a 'dud' that's just a bit too tall or has a slightly crooked this or that, or perhaps an older Jack that they're willing to sell you for a song. This will get you an animal that is welll handled, and you will be very thankful for this later. Even a pasture puff should have good manners, and you don't want to have to sedate it every time you want to trim it's hooves or brush it down.
Remember that donkeys are
a different animal than horses, despite their similarities. Their brains tend to work differently, and most will not run when they are faced with danger. This can cause them to become agressive if you have an overbearing horse or are a rough/inexperienced handler. When overwhelmed, they will freeze up rather than run as well. Donkeys also need human interaction to maintain their friendliness, and it is vital to bring them out atleast once every other day or so, even if it's just for a quick check up (for signs of playing too hard with your big boy, weight loss, etc) or a grooming session. You can't expect one to go a month in a pasture then behave perfectly when you go to catch it. Food for your thoughts.
Speaking of feed; our donkeys go through about two flakes of hay daily (no grazing; dry lot) and eat the same grain that we feed our minis (Purina Miniature Horse and Pony feed). We give one Jack one cup of grain daily, and the other gets a cup and a half. It really just depends on if they are hard keepers or not.
Unlike with minis, I'm not too worried about grass, but I would look for a donkey that has already been grass-kept, or otherwise have a small sacrifice lot to keep it in while you're acclimating it to the grass in your area. If grass tends to be extremely rich, I would look into a grazing muzzle for atleast half of the day. If the donkey is pastured with ample grass, I would start it out on only the grass, and only suppliment with hay or grain if it appears to be doing poorly.
Donkeys are a lot hardier than minis so I'm glad you're thinking of getting one to put with your horse. But once again, remember that donkey and horse communication is not the same because they are different animals. This means that your horse will still need to depend on your for social interaction, and there may be occasional spats between them; so daily once-overs for both animals is necessary. And some horses are just down-right terrified of small creatures with big honks! XD