for each floor or group of switches have a PIC monitor the switches, the PICs could be power by a low voltage AC feed in the same cable. RS485 being a balanced pair system should be immune to the AC. When a button is pressed the local PIC signals via the RS485 interface to the Master PIC which could either use a matrix to display individual LEDs or to 7 segment displays.
If you choose the central powered idea then very low powered PICs are needed if you power the PICs locally use any that will accomadate the number of switches required + the serial interface