Yep, you are right about using the seperate units, and getting a larger keyboard. It just means better quality, better playability and better longivity. As far as controllers/MIDI go:
I would say go with an rack mount synth, and use a keyboard controller.
Many are avaiable on the market and it really depends on how "keyboard like" you want it to feel, plus do you just want keys, or do you want knobs, sliders, joysticks, mod wheels, touch pads, etc. I would say at least have a few touch pads, knobs or sliders. They are great for changing effects on the fly, changing output levels, etc. Also having the ability to add a foot switch for either the keyboardist or you can be cool. Say that you want to add reverb at the exact same time you want the synth sounds to change...with a foot controller you can control this simultaneously at your will. All this really depends on how much you want to get into working with triggers and MIDI and all that jazz.
I use a Korg Microkontrol, but it prob would suit your needs (it does not have full sized keys and it has a lot of extras, such as knobs, sliders, pads, joystick, etc.). Studiologic makes some good controllers. My university has a Fatar controller and i HATE it. I have to soft-reset the controller anytime that i want to use it, and in the past few months this even does not get the controller to work correctly.
i would be weary of running a soft synth from the ibook depending on what you are using. I have crashed Absynth on the schools PM G5 using some of my synth designs (though they were using a lot of resources. As long as you dont go toooo wild, the ibook should handle Absynth, Reaktor, Reason etc., etc. I tend to push software/hardware to the limits often. I guess that somewhere deep in my psyche i just like turning all the knobs up all the way, and then throwing the switch marked "DO NOT PULL".
Like CanadaRam said earlier, in Logic, Cubase, etc. freezing a track is a GREAT way to cut CPU usage when using plugins, etc, and it is often essential. You could techincally sequence all the keyboard parts and trigger them via a switch (foot switch for instance). The reason i mention this is you could start playing with out a keyboardist in a live setting. It can be a hassle though unless you come up with some pretty creative ways to change sequences/songs (that is unless you can use another trigger to change songs/sequences). Geez i am getting off topic. Sorry i have been programming in Max/MSP for the past 6 hours. I am shutting up now.
-cameron