I have 3 150 watt HQI over my 8' long 24" deep tank and 2 were almost enough so 1 is fine lengthwise. The lights are 40" off the substrate, ~18-20" off the surface of the water. You could get away with a 70 watt fixture for a 16" deep tank if you don't have the fixture already. If you aren't planning on using CO2 and fertilizers you will have poor plant growth and happy algae though with either fixture no matter how high it is hung.
It will grow green stuff just fine. Green algae if you don't use fertilizer and CO2 and green plants if you do use fertilizer and CO2!
Unless you use the fertilizer and CO2 plants may not grow well as they can 'burn' up as they are missing essential nutrients.
Plan to suspend the light over the tank so you can adjust the light's distance from the tank. Remember the light is intense so be sure to plan for some sort of shield so it doesn't glare in anybody's eyes.