80w T5HO bulbs are 5' long, so they are probably offset in the fixture to get full coverage. They are easy to find online, but you may have a hard time getting them locally.
As far as power consumption, a watt is a watt. That fixture has 640 watts. It will use more electricty than a fixture with 3 175w MH
(525w), but less than a fixture with 3 250w MH