Casio CZ/ VZ/ FZ - Pro Series

You're a rare breed Joe. The first person I've ever met anywhere that had
something good to say about the Casio home keyboards! Just goes to show you
that everyone's tastes are different. 

I'll try to answer some of your questions. The CZ's were in production from
the mid to late 80's and the FZ's and VZ's were in production from the late
80's to the very early 90's. The 'Zs are all very different than the home
keyboard models. There are no drum beats or built in speakers for one and
where the home keyboards usually aren't very programmable the Z's are fully
programmable. So can use Casio's synth algorithms to create you own sounds
from scratch basically. Yes the Z's were considered professional models, the
VZ's even have the word professional printed on their faceplates!
Hehehehe... 

As for synthesis type, here's a listing for each Casio model:
CZ - Phase Distortion; this is really an implementation of waveshaping
synthesis that winds up being similar to analog  both programming and sound
wise.
FZ-Sampling+Synthesis, Sampler + filter and separate oscillators, capable of
small scale additive synthesis, don't really know more about this since I
don't have one.
VZ-Interactive Phase Distortion, Very very similar to FM sound and
programming wise. Very flexible for a FM synth.

You're best bet for locating these synths is probably through online used
music gear sellers or something like Ebay. The CZ's are all very common and
the VZs and FZs are a little more difficult to find but they're not exactly
rare either. 

It's a real shame that Casio was in the pro synth market so briefly as their
stuff was interesting and they always managed to come up with an approach to
each instrument that was truly unique.  Hope this has satisfied your
curiosity somewhat.

Jon