That's so exciting!! When you're get them you need to post pics!
Okay, some answers to your questions.
Water. Low quality hay (low in protein). Shelter. Farrier.
It depends on if you are going to feed hay year round, or let them out on pasture. If you are going to let them on pasture, it depends on the quality if the grass to determine how big the area needs to be. So that one's kinda iffy to answer, we'd need more details.
Totally depends on the area your in. I found my 3 donkeys for $450 each, but I was able to sell my mini jack for $1200. And all the adds I see are for $1000+, and they sell! I definitely wouldn't pay that much for one. But if that's all that's available, you may be willing to.
Depends on the donkeys. Some of them do better then others. Usually people try and stick with same gender. I've got 2 girls that live together, and there's no way I could seperate them, cause they've been together since the younger one was born. I did put my little male with them as well, and they all did fine together. So it kinda depends on the donkey.
Obesity! That's a huge one!! You need to make sure you are monitoring their feed intake. Do not feed grain! It is unnecessary, and just don't do it.
They can easily live up to 30 or 40 years with the proper nutrition and care! They can be lifelong buddies!
This forum has been super great! I was also a member of the HorseForum, and that is a great community! (I was banned cause of a change in the rules and all minors were banned
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Good luck on your journey!! I can't wait to hear about your adventures!!!