your question doesnt seem to be very clear, or maby its just me. is this lighting for just an octopus? or also corals, caulerpa, etc? size of tank and types of lighting you are looking at? I use 2x18watt NO that came in the normal black hoods. As long as the tank is sealed tight, I dont see why you couldnt use hanging fixtures. You probably want to stick to lower wattages and NO floresent bulbs so your octo will come out and play.
Ant, you can save yourself some money because you don't need metal halides. In fact, you shouldn't have metal halides, because they're too bright.
Even the day active octos don't need a lot of bright light. Fluorescents will do just fine. Maybe one 30 or 40 watt depending on the size of your tank.
You could handle the fixture in various ways - it could be a strip light that lies across a glass or acrylic covered opening, or it could be suspended over an acrylic cover. You need to look through the options and find the one that's right for your set up.