There’s a very useful comparison chart here for Sony (and other) minidisc models:
http://lib1.store.vip.sc5.yahoo.com/lib/minidisco-store/PortableChart.html
The first key thing to look for here is that the unit has a microphone input – most of the cheaper ones don’t. You need a mic input and a mic to do live recordings. The cheapest standard minidisc model on this chart that has the mic input is the Sony MZN10
http://www.minidisco.com/mz-n10.html ($239)
The other big consideration is whether to go for standard minidisc or the new Hi-MD format, which stores a lot more music on one minidisc and, more importantly, allows you to digitally upload music from your minidisc recorder to your computer. Standard minidisc players don’t let you do that, you can only transfer the music to your computer in real time, analog fashion.
In my view, Hi-MD is the way to go. You can get one of these Sony units, with a mic input, for $280:
http://www.minidisco.com/mz-nh900.html
If I were buying a new minidisc recorder myself, this is probably the one I’d get. There’s a cheaper Hi-MD recorder on the chart, the NHF800, but it doesn’t look like it will let you to transfer files from the minidisc recorder to your computer.
(Note, however, that this minidisc-computer transfer software only works on PC, not Mac. That is why, in fact, I won’t be buying another minidisc recorder when mine finally bites the dust, but instead will probably get something like the Edirol R-1)
Sony has just come out with new software that allows you to convert your live recordings to WAV files, no restrictions attached, so you can edit them, convert to MP3, burn CDs, etc…this is great news for people who were previously hesitant to go the Hi-MD route due to Sony’s proprietary software and digital rights management restrictions:
http://www.minidisco.com/digital-upload-note.html
For microphones, assuming you’re going to be doing mostly field recordings in sessions etc., I really like the little Sound Professionals t-mic:
http://www.minidisco.com/sp-spsm1.html
The other intriguing arrangement is buying a waterproof Otterbox for your minidisc recorder that has SoundProfessionals mics built right in, so you can keep the recorder in the box while you’re recording and have good stereo separation:
http://www.minidisco.com/sp-otter1.html
These are small microphones, but I’ve gotten very good sound quality from them.
If you don’t mind a larger mic, I’ve seen this Sony at a lot at sessions:
http://www.minidisco.com/ecm-ms907.html
Too big for my tastes, but probably provides better sound than the little T-mic from SoundProfessionals.
Finally, check out this excellent little gooseneck mic from SoundProfessionals, which may provide the best sound of the lot:
http://www.minidisco.com/sp-spsm4.html
Hope all this info helps.