400watt hps design help

Alright, so for now I got a 150 hps its doing alright. But I am planning on upgrading to a 400 watt hps. But everyone please keep in mind my ideas may change day to day and any advice at all is appreciated wheather its on a purchase, design, light or anything. Don't b shy. But for now my plan is to get a 400 watt hps grow tent started. I already own a bulb but no ballast or light fixture. But my plan is to build a grow box using pvc, tarp mylar, and velcro or zipper. I want to made it close to 18sq ft or so because I hear that is pretty ideal. But I thinks ima go with the rectangle style design not square so maybe something like 5'w 4'd and 6' or so high but that's just what I got in my headand this will be in a bathroom with plenty of room. It will b a soil grow. so I am seeking input, advice and answers on tent design, lighting ( I am unaware of differences between style, lighting ballasts or what I get out of my purchase ) has anyone built there own tent? any styles or tecnique? What style ballast and light. And anything else would be appreciated

well the ballast is one of the most expensive things jus like that bulb .......if urr juss growing for personal then make a 6 by 6 its better and easier