Hi....hope I can contribute a helpful opinion. I've played the Kurzweils, the Motif, Roland Fantom and the Triton. I've had a Triton for over 3 years and it's the easiest to learn of all, based on my own experience and what I've read on various online forums.
The Kurzweils are the most expensive and offer the best acoustic sounds. They are considered very complex and take a lot of "digging" to learn how to operate and program. The Yamaha Motif also does a great job with pianos and acoustic stuff, but its operating system is confusing to many people. The Roland Fantom offers a nice palette of sounds and nice features, but I preferred the Korg Triton because of the touch screen, ease of use, fantastic synth sounds, arpeggios, sampling, and easy-to-use 16 track sequencer. There are also lots of free sounds to download online for it.
You can pick up a used Triton Classic for under $1000 by the way, but if I was buying a synth or workstation today, I'd get a
Triton Extreme--loaded with new sample ROM, more memory storage and cooler features.
Rack units are also available for most current workstations.
As far as used synths go, watch out for older analog synthesizers. They are usually overpriced, suffer from unstable tuning, and don't like to be moved around a lot. I would recommend something from the 80's or early 90's like a Korg M1 or Roland D-50.
I'm not familiar with the rack units you mentioned, however, I've heard Waldorf might be going out of business, so I wouldn't recommend their gear.
[This message has been edited by Synthoid (edited 03-10-2004).]