How do I disable or remove some elements on items within certain categories? The elements I need to remove (only in selected categories) are comments, date, author and rating.

This is fairly painless:

1. In the backend, go to k2 -> categories.
2. Click on the category you want to adjust.
3. On the right side, click "item view options."
4. To get rid of comments: just before "Social sharing" you'll see "comments." Change to "Hide."
5. To get rid of date: at the top of the "item view options" you'll see "Item created date and time." Change to "Hide."
6. To get rid of author: toward the top of the "item view options" you'll see "Author." Change to "Hide."
7. To get rid of rating: 12 from the top of the "item view options" you'll see "Rating (voting)." Change to "Hide."

Good luck!

Thanks.