i'm awaiting a harman magnum stoker this friday, but i have gone the manual feed route 10 years ago with a federal coal stove, and in my experience an 18 hour burn is probable, but what heat it will be delivering with the 2 stoves you mentioned, i couldn't say. i never let the federal go that long without a feeding (12 hours tops)...but that was a different stove 10 years ago.
what i can offer you with confidence is this advice. my stove was also in the basement 10 years ago (as will the new one), and i soon learned that depending on the heat coming up the cellar stairs was impractical. i soon installed 2 - 18"X18" floor registers above the stove (with variable speed blowers in them), and it delivered as much heat upstairs as you desired. it kept the lower & upper living space balanced, and sometimes we had to open the windows a crack because it was so warm. i've never seen it 20-30 degrees below zero here in northeast pa, so perhaps you may want to increase your BTU availability in a stove, if the weather is that harsh. i hope this helps a bit, and get yourself ready for some hot & constant heat from your coal stove!