Hello Richard
You could, of course, use the three power supplies from the Argos "bundle". Since you won't be using the power amplifiers / loudspeakers at all, and the current required by the electronics is small, one of the power supplies would be perfectly adequate for all three keyboards. You'd need to have three power plugs attached to the output, of course. The easiest way to do that is with Maplin's "2.1 power extension lead set", order code JR94C.
As for switching the keyboards on, it's easy to find the circuitboard tracks that lead to the power buttton; scrape off the lacquer and solder a bridge between them. The keyboards will come on as soon as they receive power, and it doesn't matter that the button remains shorted out while the keyboard is in use.
I used simple push-to-make switches for my pistons, so I can't help you with the details of "real" pistons. Daniel got his from Kimber Allen.
Ella Pounder was a distant relative, and taught piano in Middlesbrough for many years. I wasn't aware that she played the organ, though.