Can be uno or equivalent (328P based) on both ends.
RS485 is separate chip, like a MAX488.
How many relays, what kind (SPST telecom relays for example), will they need a coil driver like ULN2803 or high current shift register like TPIC6B595?
That'd be a fun little board to make up, similar to this:
This had 5V coil telecom relays and RS232 for 1 com channel and a USB/Serial adapter for a 2nd com channel. One ATmega1284 for the interfaces, so just the 1 IO chip needed - replace that with RS485 driver and you'd be golden!