I would suggest you ditch the MD entirely, and stick to Ipod or Ipod mini.
Here is why,
Current generation of NetMDs (N420D, N520D) don’t even offer “Line In”. low end unit of previous generation of NetMD (NE410) don’t have it either.
What other options left for you. (N510, NF610, NF810, S1) they do offer “Line In”. with exception of S1, I bet you will have hard time finding the first three models in stock at your local stores.
Since you would like to record from microphone (rarely), then you would either need a model with “mic in” (ie NF810) and a mic; or obtain a battery powered mic to be used with last generation of NetMD models (ie N510, NF610 thru their "line in").
Unless Sony release the “wav”converter proggie, afaik, it is impossible for any NetMD to upload songs “Digitally” via usb > sonicstage.
At this point, if you still prefer NetMD over Ipod. Your best bet is Ebay, coz, you might get a good deal on last generation of NetMDs (New or hardly ever used) for cheap.
Now move on to Ipod. (Since you have a Mac)
Numero uno… Xfer speed. we all know that usb1.1 just can’t compete with firewire.
Both “Line in” and “Mic in” can be had as adapter, and it is accessory for the Ipod. So the recording from your old cassette/radio walkman, class lecture should have no problem with Ipod.
Design wise, Ipod sure can fit into your pocket, and look much cooler than MD..bar none. As far as durability goes, if you babied it pretty much, and not let it drop out of/from (your hand/table/desk). Otherwise, Ipod will be just durable as any MD.
The only thing you won’t like about Ipod is it’s battery life on the unit. Other than that, I think Ipod outperformed MD in terms of ease of use, huge capacity for songs or files, and fast Xfer speed. IMHO.