I have to disagree - A bathroom in an old warehouse or office building would actually be too large for 5 plants. Even a private bathroom would be big enough for 5 plants. And if he's lucky, it'll be lined in white tile. That would be ideal. I agree that you can setup without walls, but why waste the light? Also, you stated setup without walls, then build a frame to hold the lights, its much easier to just attach the lights to the already created walls.

You guys are talking like he's going commercial, he isn't, he's doing a few plants. Even in OLD warehouses, there were offices and bathrooms. Utilize what you have. To the OP, this is exactly why I said your question was too broad. For 5 plants, you can go with a 400-600 watt light for flowering, and a 125w cfl or a 150w metal halide for veg

Good luck!