Congrats. Yet another advantage of a digital ballast is they will run both MH and HPS if you use a conversion bulb. At least all the ones I looked at did.

I don't think I ever saw inline fans at home depot, but while you are there pick up some ducting that is straight duct pipe, and not the coily clothes dryer stuff. It wil make a world of difference in the amount of air that can be sucked/blown through. Also minimize turns or corners as much as possible. They really cut down on the air flow.