Hi Stu,
I can can only recommend, from good personal experience, the Sony PCM-D50 with it's Optical I/O feature. I have transfered nearly 200+ Minidiscs, in SP Mode Only, without a single hitch. I will tell you though, due to my own ignorance at the beginning, the D50 was not picking up MD Track Marks. After a brief call to Sony Tech Support, all that needed setting up on the D50 was the Sync Feature, smooth sailing ever since.
Now you may think I'm nuts... but sometimes I like to transfer the otherway from D50 to Minidisc for kicks. If you ever should feel the urge... remember to use the Time Machine (6sec buffer). If you don't, the very beginning of a MD Track may not be recorded on your MDS-JB940, due to data stream not activating immediately the Optical Record. The buffer makes up for it!.
Anyway... I know this does not meet your $300 range, but may well be worth it, since Sony advertises their Optical feature as put there to satisfy both DAT and MD lovers, alike. Maybe Ebay?
Best of luck on your choice.
Gordon