If a mouse works with a G5, it works with an iBook. As far as peripherals go, if it works with OS X it works with OS X, regardless of the computer, and as far as I know, any USB mouse you buy will work with OS X. GarageBand does come installed on every new Mac as part of the iLife bundle (also includes iPhoto, iTunes, iMovie, and iDVD (if you have a SuperDrive). It sounds like an iBook will be a great computer for you.
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