Since Rockbox plays MP3s (among other formats), it will definitely be mp3-compatible. Most hard-drive based players these days use built-in batteries, though they can be replaced with comparible batteries (possibly higher capacity in the same physical dimensions) and the proper tools if necessary. As for space, the iPod 5.5G 80GB is the highest capacity without replacing the drive with a larger one. That may be your best bet if you don't want to replace the hard drive. If you're willing to replace the hard drive, you may prefer the Gigabeat F, as it's got a much more powerful processor. Either way, you'll probably have to look on eBay for either DAP.