Jerkash, your question seems to be on what to do with the water at end of season. Not sure about your brand, but with my Central Boiler a corrosion inhibitor water treatment was put into the furnace on the first fill. My furnace came with a water test kit, which we are to use by testing water at suggested 6 month intervals. It balances nitrite, protects and eliminates any rust in the water jacket. Owners manual recommends not draining furnace in off season as it it protected. If drained, it would lose that protection as air would then come in contact with the unprotected metal causing surface rust inside. In my Classic I never needed to add water or water treatment until after 4 years of use and the water was still crystal clear. Had evaporated just about 1 inch of water and added about 1/4 jug of treatment.
As far as firebox, scrape all creosote lose from firebox walls and bottom. I shop vac then wipe down with light coat of new motor oil. Thow a bucket over the chimney. I even scrub down furnace front with Simple Green. Furnace stays great looking for years of longevity and service.