OK. 500 meter is almost half the max distance of a RS-485 network. So termination is better to be used. The UTP is indeed unshielded. If there are no very strong interferences I see no reason to use other cable. The twisted pair has the property of self cancelate any noise presented on both the wires at the same time.

The opto isolation is really necessary if the two nodes are at different voltage referenced to (common) ground. If this is the case the do use isolation. Otherwise connect directly and the ground with 100 ohm resistor to be safe.

For the role of this resistor I direct you again to Axelson's book.

Ioannis