At 18 inches depth, I would be thinking of 6x 24 inch T5HO w/ individual reflectors, or a single MH that will be just a bit dim and the ends. You may get by with 4 T5s but I don't think that 2 would do.
Oh, and as far as PC lights, they cannot be reflected as well as T5 because of their shape. A T5 can have a reflector wrap around it and reflect much more light into the tank. T5HO will also last 12-18 month and a PC is really losing a lot of PAR (a unit of usable light) after 6 months. THe bulb costs are about the same so PC lights can really get expensive on upkeep.
an older friend of mine kept a bubble under pc's for a good amount of time. I lived, but did not thrive. t5s will be the cheapest alternative for you IMO. a 4 bulb unit will be more then plenty for your setup, and also gives you plenty of options for other coral species etc...
I would look for a 250 watt metal halide pendant with a ballast that someone is selling used. I am currently running a PFO pendant with a Phoenix 14K bulb over my 54 gallon corner tank. If you look at the link in my signature line, you will see where I posted pictures of the tank with the halide over it.
From what I read you supposedly need 3 - 5 watt per gallon, I run 1/2 of the recommended power and have corals growing like mad. From diving I see the light is way dimmer than in my tank very few feet down so honestly I don't really understand the massive wattage needed. But maybe some close to surface corals need it, maybe some corals from crystal clear shallow water need it.
I use the ocean growth blue and high kelvin fluorescent 50/50, as long as the inhabitants are happy and grow I see no need to burn loads of money on power just because.
I would recommend that you try it out gradually. Just get some cheap fluorescent fixture of industrial type, try 4 tubes in a length which fits nicely the width of the tank. put in 2 ocean growth blue and 2 say 6000 - 10 000 kelvin tubes. See if the stuff in your tank is happy and grow. If it is, leave it. Cheap to buy and run, beats burning money for something you might not need several times over. If there is one coral unhappy, sell it and keep the ones who are happy.
The corals which grow like weed in my tank is bubble coral, elegance coral, elephant ear coral and several spices of button corals, they look very nice and are easy to keep. Should I go for a type of coral needing twice to four times the light I would go metal halide.
Try, observe, adjust seems to my setup to become successful.