MH doesn't necessitate running a chiller. I started off with t5s over both of my tanks and I don't think you'll be happy with them.
I have a clone of the Odyssea fixture you linked to. Mine is th 48" version that has (2)250w mh in it which should be fine for your 125g tank (mine is a 90). If you want a 72" fixture that should be graet. The hanging kit at the bottom of that link is great, it let's you move the lights close or farther away from the tank to control the light, acclimate corals, or adjust when you get new bulbs. I did add a small fan to the end of my tank and I don't have any heat issues.
Oh, BTW, Icecap has gone out of business. I've had their ballats and did some DIY work for my old canopy with them. I liked the ballast and product but would be a bit worried about buying them now since Icecap isn't around for warranty or servicing them.