Just some small update: I have build up the circuit on a breadboard and tested all the components in software. So far I'm pretty satisfied with the results and think I will go on with this. A schematic will follow soon, so far just a pretty picture of the board all wired up and with blinking LEDs...

BTW, I had some thoughts about the power supply for the project and came up with some very nice idea ... more about that will also come soon.