Donkeys are pretty hardy and low key, so they're inexpensive pets to have. It's pretty much the same as a horse: fresh water, good food, some attention, and they're happy.
They do enjoy company (do you still have horses?) so having a friend around would be good. Their coats are way different than horses, their hair is coarser and thicker so I use a shedding brush to get off all the debris (and there's a lot, because they love to roll!). Their hooves are also different, my donkey doesn't need trimming half as often as my horse does; mine is happy with a handful of sweet feed and some hay; he does have 24/7 access to pasture, but he has a shed for shelter and a tree-line for shade. As far as training, they're smart but can be really stubborn, so it's important to be patient and take your time. Are you training him for anything, or is he just a pet? 