If you have less than 10 or so users, id say go ahead and self host. It's not terribly resource intensive at least not on my personal instance. I use it to test posts, solutions that will eventually make it's way into a pr, or just experiments and that can (and does) run on a pi.
Very very little. It's a billion tiny little bits of text, and if you have image caching enabled, then all those thumbnails.
My personal instance doesn't cache images since I'm the only one using it (which means a cached image does nobody any good), and i use somewhere less than 20gb a month, though I don't have entirely specific numbers, just before-lemmy and after-lemmy aggregates.