You don't have a physical backup unless you burn one .. but it really isn't needed unless you have no internet connection.
If you get "real" CDs, you can rip at any resolution you wish. If you get MP3 CDs, then there is no difference typically in the files. Most are at 320K constant bit rate.
I use idjpool.com (Illinois Record Pool) and get more music than I could ever use. Only dig (if you can call it that) is a limited selection of non-mainstream (reggaeton, soca, jazz, etc.).
I typically download once a week and get almost 1,000 files.