If it's an MP3, it'll work on an iPod. What you do once you've downloaded the MP3s (keep track of where they go) is drag them into iTunes when it's open -- specifically, over top the "Library" section on the left. Once they're in the library, they should reach your iPod the next time you sync it.

You can also create a playlist and drag the music you want to burn to CD into it. That gives you a button in the interface that will let you burn the disc. Keep in mind that you can go into iTunes' preferences (in the Edit menu) and click Advanced to help set the kind of CD you want to make, i.e. a regular audio CD, an MP3 CD (if your Bose unit can read MP3s natively), and so on.