you can find lots of info on light specs on this site. just go to the equipment forum and do a search on lighting and you will find more infromation than I can give you. I am by no means and expert. It is gonig to depend a lot on the type of corals you want to keep. With pc's or vho's you would be able to keep soft corals (which zoas fall under I believe). For some of the lps you would be able to keep under pc's. Your best bet would to go with a mh/vho combo and cover all the bases. For a 180 I think 250 watt mh's would work. You want to try to have one mh bulb for every 2 feet of tank (i think) You then would be able to keep clams, sps, lps, softies. Better to make the investment now than have to upgrade later.
Currently I have 3 clams, frog spawn, bubble corals, sps of diff types, a yellow figi leather, mushrooms, and they all do well under mh's. The only thing that I have not do so well under the mh's was a rock of white mushrooms. They fell apart only days after having them in my tank. I even tried to move them to my Nano with the pc's but it did not help.
If you go with mh's expect to have to deal with heat. I have a fan in my hood that does the trick for me. If I had the money to invest I would get a chiller but there are too many things inline before it, so it is not really an option.