The higher it is from the surface of the water the less heat transfer to the tank. At least 6 inches from the water surface is recommended. The fixture your talking about has a hanging kit (extra $) you can put on it so you can adjust the height. I'm looking at it right now in the catalog.
The fixture only comes in 70 or 150 watt from the 20'' to the 24''.
Have you seen the SunPod Pro HQI? Same thing just more output. The 24'' has a 250 watt 14,000k MH
which would give you about 7 watts per gallon in a 36 gallon tank. But it is an extra $100 for the "pro" fixture.
O and both types of fixtures have built in fans.