<< should I drain all the water out of the furnace? >>
Why bother? The water in your boiler is as oxygen depleted as it can be. Adding
more unboiled tap water later will only cause more rust buidup if that is your
concern. Look at any old heating plant parts in a scrap yard and you will note
that cast iron is quickly passsivated from corrosion by the continual recycling
of the water.
The system manufacturer will have literature to guide you if your service
people can't provide one. Good luck.