I think if you plan on using the same solenoid for selecting a soundboard and then for individual pipes you're going to have massive timing issues, not to mention space.
I'm not suggesting you should use the KA units, but a browse through their catalogue might give you some good ideas.
Hauptwerk provides an interface to stops exactly the same as a real instrument. Each drawknob unit connects to a midi scanner board (which in turn drives the knobs for combination action). I'm going to be making a lot of units. Voice count depends on the organ sampled. I'm basically building entire drawknob units from scratch, matching the input/outputs of a regular knob.