I would research corals. Go to a website like Live Aquaria or similar and decide what kinds of corals you want and then choose your lighting. If you can't decide, go with the strongest lighting you can get, so you aren't limited. it's impossible to say how many watts you will need, because no two fixtures are created equal. You could also look on this site and find someone with a similar tank that you like the look of, and see what lighting they are using.