Mark first off you may want to refer to this doc from gecko.
http://www.geckodrive.com/photos/Step_motor_basics.pdf
Also bobcampell designs has some info on power supplies but I have lost that link I just saw it in another post today so check the other threads in this area.
Those drives should suffice in either way. I would highly reccomend that you only use only one rectifier for a number of reason, but simplicty is most likely #one, and larger componets IMHO seem to be more robust. You also limit the chance of a crash if you have a PS failure, since none of the axises will be working.
Are you refering to full load voltage drop(ie the difference between full load and no load) or are you refering to secondary output? Keep in mind theory and reality are two different things you never hit full load with all 3 axises on a standard cnc setup.
Don't over engineer/think this its typically very simple, KISS applies to this.
Chris