Well, I've decided that the unipolar 5 wire motors I will drive through the original darlington IC:s (Brother) and the 6 wire motors I will drive as 4 wire bipolars using Sparkfun's EasyDrivers (Canon). The idea of hacking into the Canon LCD-keyboard-character handling is way beyond what I originally intended to do with these typewriters, so I'll probably just leave it.
The Brother seems to run on 12 V. I'll probably just reuse its own power unit. I just have to figure out if 12 V is too much for the Arduino. The Canon uses some 22 V. I have a spare laptop power unit with 18 V 3A, which I intend to use. It will provide voltage and current for the steppers. The stepper drivers in turn will provide the voltage regulating for the Arduino.
I still don't have a clue on whether both typewriters will turn into plotters. If I don't get them to perform half steps or micro steps, I will ditch them.