If you can get that Kurz for under $600.00 then I suggest you go for it. You will stil need a MIDI keyboard controller for software synths. A kurz is as good of a MIDI controller as any. It is also a pretty powerful hardware synth and it is much better to have best of both worlds than just go either hardware or software. The sweet spot is in the middle.
After you've got your Kurz and you get pretty tired of just playing it and trying to work with it's internal sequencer you'll probably want to add a MIDI interface and a sequencer/audio program (like Logic, DP4, Cubase or perhaps Pro Tools LE. All will work nicely on your iBook. As long as you run OS X
).
Getting some type of a sequencer aplication at some point is going to be a must. Most of them come with built in software synths (I know that logic Pro, Cubase and DP4 do) and of course you always have the option of adding 3rd party software instruments.
Speaking of which I really like NI Reaktor, Absynth and FM7. Reaktor probably being the most powerful software instrument due to it's completely modular structure and ability to go in any direction synthesis wise. It lets you build and design your own synths. Truly limitless. Much like the Kurzweil but not bound by the same limitations of hardware.
If you combine using Kurz K2K and NI Reaktor youve pretty much got the sounds you're after. Adding other synths will never hurt since all synths sound different and have features that are unique.
I'll name a few of my favorite one-trick ponys:
1. NI FM7.
2. Apple ES2 (comes with Logic Pro).
3. Arturia Prophet V.
4. Waldorf Attack.
5. Waldorf PPG2.V
6. NI Pro-53
7. Gmedia Oddity.
8. Arturia Moog MOdular V.
List goes on but i'm just trying to name synths that I like to work with.
Anyway. google for all the stuff I've mentioned here. download demos. check 'em out. I think all of it will run on your iBook just fine. Might need a bit more RAM (at least a total of 1GB).
-ED-