18 AWG is about 21 mohm/meter
500 m is about 1 ohm
a pair is about 2 ohms. so the wire is not a significant voltage drop
use twisted pair to reduce external noise pick up
or better, twisted, shielded pair like belden 8760
if it will be exposed to the weather, you should use something with a more weather resistant outer jacket
Texas Instruments differential line driver SN75158 and differetial line receiver SN75157 are both available from digikey
they are designed for RS422, with a max length of 1500 m