36x18x18 is a 50 gallon tank, so if you want 1.5 watts per gallon you want about 75 watts of AH Supply type lighting, sitting on top of the tank. If you use T8 bulb lights, in a white painted hood, you would probably want about 100 watts. If you used T5 fixtures, with each bulb having its own reflector, you would probably want about 50 watts, unless you suspend the light higher above the tank, 6 inches, for example. Then you would probably want the T5 fixture to be about 65 watts or so.
See, no one value for watts is good - it all depends on what type of bulbs, what type of reflectors, and where the fixture is located. The advantage of getting more light than you need is that you can always raise the fixture higher above the water to reduce the intensity, but with a light that is not intense enough you can only add more lights to get more intensity.
Some of the plants you could use would be crypts, anubias, ferns and mosses.