About the steam and the ridge vent, regardless of the cupola or not, some steam will vent thru that type of ridge vent. A hood can all but eliminate that. I highly recommend you use a sheet goods type under the steel. If not the steel roofing will still sweat some and rain inside. I know, it has a way to always drip down the back of your neck. A hood all but eliminates that.
How large of an evaporator do you see in your future? If counting on a cupola to vent the steam, make the cupola that square foot opening. If you never see one larger than a 2x6 or maybe a 3x8, make it a 12SF or 24SF opening.
With an RO a 2x6 can do lots of taps worth of boiling, a 3x8 can do double that.
Dave Klish, I recently ordered a 2x6 wood fired evaporator from A&A Sheet Metal which I will be converting to oil fired
Now have solar, 2x6 finish pan, 5 bank 7x7 filter press, large water jacketed bottler, and tankless water heater.
Recently bought another Gingerich RO, this one was a 125, but a second membrane was added thus is a 250, like I had.
After running a 2x3, a 2x6, 3x8 tapping from 79 taps up to 1320 all woodfired, now I'm going to a 2x6 oil fired and a 200-425 taps.