Sidebar "within" content area?

I’m currently redesigning our site’s theme. At the moment, I require my writers to start each post with a large image or video, and then start the body text immediately after, and finish with a source link.

What would be the best way to add a sidebar for post info and links “within” the content area, below the pic/video but next to the text? Example:
http://www.cnn.com/2010/TECH/social.media/12/30/facebook.beats.google.cashmore/index.html
I assume I can’t “split” the content area, and that the best way to do this would be by assigning the pic/video AND caption to a custom field, call that, then the sidebar, then the actual loop.

My questions are: 1) I’m not too familiar with calling something like a custom field, and specifically before the actual post has been called. Any suggestions?
2) Last time I tried to use custom fields, my writers were either confused or it just became easier to forget than “built-in” fields such as the title, tags, categories, thumbnails, etc. Is there a way I can make a “custom field” built in, so that like Post Tags, Categories, and Thumbnails, it has it’s own dedicated section?
3) Since WP is smart enough to convert a YouTube link into an embedded video, is there a way I can set this custom field to take an inputted URL and know to either make the header an image (if it’s an image file) or an embedded video (if it’s a video URL)?