All You would need would be a small expansion tank with even a radiator cap for a pressure relief. When I was working @ Freightliner, the small engines, (Cummins 5.9 and 8.3,) used an expansion tank separate from the rad as a fill pressure relief, with a radiator cap. It even had a low water level site glass.
This could be placed at a high point in the system for maintenance.
Because the system would be completely within the bus, antifreeze would not be necessary, except some form of coolant treatment would be necessary to avoid corrosion of the heater pipes.