i used the ESP32 beacuse of all the possibilities it has. But its a little bit more complicated to use it than a simple Atmega/Attiny. You have to shut it down, send him to boot mode while starting and then reset after a sucessful upload to bring it back to the execution mode. And it needs 3.3V to work, which can be aquired by using a linear voltage regualtor and feeding it with 5V (yeah you CAN use higher Voltages but otherwise its getting pretty hot...).

Writing on in a few hours...