At 18" deep I would recommend 250w. I've done 400w on tanks about that depth or shallower, and in the end it took longer for SPS to acclimate to the light, and was more expensive to run. It was also quite warm both for the tank and the room itself.
A good quality reflector will take you further then a less good reflector with the same bulb & ballast set up. At 30" wide (I am assuming) you could get away with one bulb and a good reflector, e.g. luminarc mini + bulb & ballast of your choice. However you mentioned wanting to go with an all in one set up, and I don't know enough about them to comment.
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