MH lighting is very expensive and takes a bit more to maintain than most other lighting.
If you have a canopy you can just set up a metal halide retro kit inside of it; the kits cost around $150-250. This may be a really bad estimate. You need a special ballast for MH ($150), the bulbs themselves (2 250s for a 55 gallon, $45 each), and anything else that is needed which I cannot think of.
Like I said, there are retro kits that have everything you need to set up inside of a canopy. If you don't have a canopy getting a hood for MH can be upwards of $500-900.
If you don't mind being kinda restricted on lighting, just go with a power compact (PC) for now. You can keep many corals and most anything else you need with this lighting. It is much, much cheaper and covers most of what you need.
I was going to go MH until I looked into pricing more. PC is for me, at least until I get really situated into the hobby and want far grander, challenging things.
Moonlights I am not very familiar with; if you go to the DIY forum you can see how to make moonlights for under $5. I believe that you do leave them on all night or put it on some kind of "realistic" cycle. Dunno, don't take my words for it.