If you want a riding donk you will need a large standard or Mammoth (crazy expensive). Go with a breeder that shows, as most of their donks will be of great conformation. Funnily, it's one of the last equine endeavors that is very low on politics in the show ring.
Donks can carry up to their own weight in goods, but I wouldn't make one carry that much personally..
You need to find a halter that fits. Some use Arabian halters, some use pony, some use cob halters.. it's kind of trial and error.
They are pretty similar to horses are far as foot trimming (though not as often usually), worming, and food. They do need less food than most horses though, and should be dilligently watched so they don't become overweight.
Donkeys are smart. Far smarter than any human. To work with a donkey, you need to get them to trust that you are an equal or better. It's hard work. They are very unforgiving of mistakes. They are also one of the funnest animals to own. I always recommend a pair, as some horses don't "click" with donks, and some donks don't with horses. Having a pair (esp. From the same person) ensures they will have a friend.