There are a few things to consider:
What kind of music are you recording? You probably do not need an external power supply ("battery box") for quiet, acoustic or minimally amplified music (most classical, folk or jazz). But you want to tape rock and pop concerts, then a battery box with bass-rolloff is advisable.
What medium do you want to store your music in? MD is still almost de-faco in the amateur taping circle. Hard-disk recording (mp3 et al) has not caught on yet, simply because there is NO satisfactory recorder in existence (not enough consumer demand, I presume). Hi-MD is an attractive option, but this confines yourself to Sony machines, which is not a good thing IMO.
Insist on a SHARP MD recorder: Sharp has a big advantage over Sony machines in that you can adjust the recording level without stopping the machine. Even the Hi-MD units from Sony require you to press Pause if you want to tinker on the recording level.
"Digital recorders" with a mic on the chasis are ONLY good for taping speech; for music you have to use a separate mic with a cord -- the mic will not pick up the noise generated by the moving parts of the recorder, and you can fumble with the recorder without disturbing the mic and ruining the sound. I much perfer a small CARDOID (i.e. unidirectional) mic capsule to an omni-directional; you pick up far less audience noise.
Go over to
www.minidisc.org for more tips on audience recording.