I dont think a single MH is going to have enough spread to cover a 6 ft tank. If you look at most fixtures, they have 3 MH's with some T5's for supplemental light
I also have a 180, the most expensive part about a 6 ft tank is the lighting, its far more costly than anything else needed for the tank. I think you'd need at least 2 but probably 3 MH's for a 6 ft tank. If you just run MH you wont have T5's for supplemental color (unless you have a canapy that would let you run both, or you went with a fixture that runs both)
I couldnt beleive how much heat my 3x 150W Metal Halides were putting out. When I switched to LED's my tank temp dropped 3 degrees, I need to get some larger heaters to make up the difference
I have the 48" version of this same lighting fixture on a 75g. I have had no problems at all. The output is great and you cant beat the price anywhere.