Hi Aurel,
MyDoorOpener will work with most types of Arduino hardware. There are no hard constraints
except on the TCP/IP stack because we rely on the
WebDuino library. So YES, you could use your existing
Arduino Uno + Ethernet Shield and that will work. Your challenge will be to have a Relay board stacked
on top of an already stacked Uno + Ethernet ...
So NO, you will not need the USB2SERIAL kit if you already are able to upload code to your Arduino Uno
using your existing USB cable.
For the Relay board, on that front too, there are no hard constraints. All I/O pins are configured using constants
in our documented .INO file, so you can change whatever's in there to suit any of the Relay boards available
on the market today. Some people have used the SeedStudio relay board, but we've always worked with
the DFRobot relay boards. Both (and any other) will work.
Best regards,