It really depends on your purpose. If you are recording yourself only to hear the quality of your playing (pianist right here) and nothing else (no burning to CDs, no sending a copy to your friends), then any MD recorder would do.
Information on many Sony MD recorders (except Hi-MD) available at:
http://www.minidisc.org/part_Recorders_Sony.html
If you're just using your MD recorder purely for recording, then older models are good enough. If not, and instead if you want your recorder to also be your handy portable player (and have a few mp3s and/or wma's you want to hear on the go) then Net MD a possibility. If you want the biggest capacity possible MD recorder, then the recent Hi-MD is your answer... but neither Net-MD nor Hi-MD can really forward/rewind in the cassette style, and only offer a CD player's "skip by track" method.
**NOTE** Net-MD recorders disallows transferring your recordings back into your computer... well technically you can using the "analogue trick", but it degrades (and alternates) the quality of the audio quite a bit, which is probably the nightmare of every musician.
If you want to transfer your recording back to the computer in its original format, I suggest you target the Hi-MD players. Although Sony seems to disallow burning the uploaded audio on to a CD, or even extracting it for other purposes, you can record a near-clone of your computer's own audio output providing you know how to adjust the windows mixer settings properly and have a decent sound card.
In addition, some claim Sony finally disabled the Hi-MD's file protection on Sonic Stage 2.1, but the release date of this software or whether the long-craved-for protection-removal update would be included...
Edit 07/21/04: Sony just might fix the protection problem... http://forums.minidisc.org/viewtopic.php?t=5417
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On the note of the microphone, I'm rather inexperienced with this. I personally recorded many live performed music with a Sony ECMMS907 mic... it's pretty good except I would prefer a bit a higher response range. (100Hz - 15KHz).