Search in this page for DS18B20 : Arduino Playground - InterfacingWithHardware
Perhaps you find some new ideas.
I don't know the ModBus. Does it use RS485 ?
http://playground.arduino.cc/Code/ModbusMaster
There are expansion shields with RS485.
If you could make them work with ModBus, you have a reliable connection.
If you have ethernet cables, you could use an Ethernet Shield, or an Arduino Ethernet. But using ethernet is slow, and it doesn't handle slow or bad connections very well.