I saw some mindisk drives on Ebay a while ago and I've been kicking myself ever since because they only went for about $120.
As far as internal dries go, your going to have a really hard time finding them and your going to pay a butt load.
If your interested in recording digitally I would reccomend getting a regular mindisc stereo unit - check for the digital I/O's and such.
Sony stays away from anything internal. They want their ATRAC compression to stay away from anywhere people will be able to get their hands on it and decode it. Thats why all the encoding is done by hardware.
Until Sony fixes OpenMG, I wouldnt suggest NetMD mindisc player/recorders
Thats all from me
peace