Well, you won't need this PC Link thing if the MD desk itself has a direct USB port. The link above leads to a signal converter, that converts digital content from the USB port of your PC to light impulses that go through a regular optical cable. AS far as I#m aware, you can hook up any MD/CD-Recorder that has optical inputs. Important thing is that your MD Deck has to have optical inputs, some only have coax!
Please post what kind of MD deck you're buying, the way it looks to me is that is does not need the conversion, it simply takes digital data from the USB input, runs it through the ATRAC3 encoding scheme, and stores it on the disk.
About Minidisk audio storage: A Minidisc is certainly closer to a HD than a CD. The audio data is written in a compressed format called ATRAC3, on a rewriteable magnetic (optical) surface. That's what seperates it from the CD, it's rewriteable, but a lossy format. The structure itself is rather CD-like, with the addition that you can alter it afterwards.
A TOC (Table Of Contents) is created with all track index marks, track order and all that at the end of the recording.
This TOC can be altered so you can divide/merge tracks and even "move" tracks (TOC just tells the player to start at minute 30 for track 1, minute 3 for track 2, min. 45 for track 3 etc...). The audio data itself is left unaltered, all changes are made in the TOC at that level. You can of course delete parts/tracks in the middle, which DOES change audio data and TOC, but it only show up as blank space if a certain limit is reached (e.g. the blank space in between is big enough). This "defragmentation" of your MD is of course possible, but you should try to avoid it, just as moving tracks after recording since drive mechanics are streched without need & sense; record properly the first time.
But as far as mobile audio is concerned, MD is a thing of the past. Those new mp3-cd players are the killer since you can put hours of music in higher quality than ATRAC3 on them. The lastest version of ATRAC are really good, but a well-done mp3 encoding with LAME or FhG is at least on par, if not even better. See hydrogenaudio.org or r3mix.net for details. Especially look in the forums, these are quite up-to-date. Post your questions on mp3 to audiofora.com (that's r3mix.net's message board with lots of helpful and very skilled users.