I previously envisaged using an Atmel ATTINY85 8-bit MCU due to the small size and really low cost, but then realized it will not have enough I/O pins and may be too underpowered to manage everything. I've now decided to use the beautiful 32-bit ATSAMD21E18 microcontroller which is available on a few different development boards and has a lot more Flash and RAM available. I will begin by using an Adafruit Trinket M0.