For reference you can completely delete the previews folder and still be fine, the only thing you really need is the catalog. Previews folder is just that, a folder that holds previews LR creates so it's quicker to view images, these are generated automatically when you open an image.
I don't use the LR backup feature, I automate backups to 2 external drives. I only copy the catalogs which makes it much faster. I also split my catalogs by year and sometimes events/holidays, depends how you like to organise things.
I'm on the fence about LR5, not enough differences over LR4 to be a game changer but things like the new spot removal tool is handy, it also seemed to be a bit faster loading images to work on. I may get it after the next patch.
On a side note I found an interesting tutorial video, it's probably a fairly basic video for some, more geared for new users with LR but I did find one or two tips in it. It's for LR4 not 5 but the fundamentals are the same. Best off watching on YT in 720p.
The B&H channel is really good, there are lots of 1 to 2 hour videos from pros which i found really interesting, they cover most fields and subjects, either howtos or more on how they work.
http://www.youtube.com/user/BHPhotoVideoProAudio/videos