Well the the trigger's been pulled and a 67 litre (~17.7 US Gal, or 14.7 Imp Gal.) tank was ordered - (or back-ordered actually) electric as there is already service to the space in question. Gas was run in last winter so we could heat at least part of the house with a gas fireplace, but was ruled out for the back garage.
We're chlorinated here so I'll go with the 125F setting. The money saved not going tankless has already been spent if you talk to SWMBO. (drainage and water so I would need a water heater...)
Got the sleepers down (level floor, yay), the subfloor down and about to start the partition wall...
So much closer to having a darkroom again after two years of NOT.
Tanks to all of you!