Suggestion, add RS485 
TTL Serial, even at 4800 baud, is kind of a hack.
And resetting an Arduino because of millis() overflowing is definitely a hack / bad coding. Just read Blink without delay and do:
- use an unsigned long for storing millis
- always check like millisNow - millisLast >= interval
And you have zero problems. Now you just build a Goldberg machine
And for cable, cat5/cat5e/cat6