I had a similar situation with my darkroom - I was worried that since the instantaneous power drawn by a tankless system would create a big power draw, and potentially dim my enlarger bulb, I opted for a small conventional heater. The heater that I have is ~5 or 6 gallons, and is powered by 110V - it sits under a bench and seems to work well. If I had gas in the garage, I probably would have opted for a gas powered tankless system.
I don't use a lot of hot water in the darkroom, mostly just to mix with cold to bring the temp up to 20C, so a small capacity heater works just fine. I turn off the heater when I am not using the darkroom.