|Yes, you need an operating system. How else are you going to access the files or play them or rip music from the CD's?|
For what you want to do, would require very little power so you could probably find an older laptop on Craigslist for a good price. I used to use an old Pentium 1 500MHz laptop for that purpose and it worked just fine. Depending on how much music you want to store on there and the bitrate you encode the songs at will determine how big a drive you need. How many CD's do you have?