I put in an order to Digikey for $70 (!) of electronics, including some high power MOSFETs for the solenoid drivers:
, some parts for making molex connectors,
, three CPU boards (basically cheaper, more powerful, lower level Arduinos),
...and a ton of stuff for audio output, including this surface mount op-amp, which turned out to be *really tiny*
I made some nice compact solenoid driver boards
Looking at schematics for actual machines they've got all kinds of weird stuff, however just a MOSFET, two resistors, and a line to the CPU were enough to fire the solenoids just fine. I wish I had some way to turn them off in case the CPU locks up while one is turned on, but I don't know what that would be, and as far as I can tell 'professional' machines don't do it either.