The original files, and the catalog file, are all you need to back-up
I don't keep a lot of edited copies of images. They aren't really necessary. I keep my master images (all raw files) and use them for browsing in Lightroom and for printing. Sometimes there are TIF files when I have gone to Photoshop to make further corrections. But other than that, I don't keep copies. I make copies for e-mail or to send to a lab, etc., but once they've been used I delete them. So backing up the master files and the catalog is really the most critical, in my opinion.
thanks for the help