I've made a pretty decent bit of money buying parking lot metal halides and converting them into aquarium lighting. They work fine, you have to make sure the bulbs use the same kind of starting mechanism the ballast puts out. There are probe and pulse start bulbs.
A 175 mh uses a m57 ballast and capacitor. A 250 uses m58 ballast and capacitor. A 400 uses m59 ballast and capacitor. Most MH bulbs use a e39 mogul socket.
These are numbers for a magentic probe start ballast.
A good alternative is to find parking lot lights with these numbers and wire them. I have $150 or so in 2 400w mh on my 110.