1200w is the way to go, that is what I am set up with. Keep them way under 6ft though! I have tried that with 600w and it works awful. 5ft is the max, 2-4 ft is perfect. But it's good you are using R/O though! All your beneficial bacteria we pay so much for is rendered useless by tap water, it is full of chlorine and different chlorates. This doesn't seem to be so bad on the East coast, but it really is a must here on the west coast.
A 1200w grow should take up about a 3x8 space for plants. Do not spend a bunch of money on expensive insulation unless it is IR block. I have discovered by using a double layer of plastic, I get FANTASTIC insulation, and get perfect room control, doesn't matter if there is 4ft of snow outside, or if it's pushing 90 degrees outside, everything works the same year around.
Get thick plastic, at least 4ml thick, higher quality panda film should work, but I usually use something thicker, black plastic on the outside of your stud, white or clear plastic on the inside, I use clear that I coat with mylar, using spray on glue. I use really high quality zippers from ZipWall for my entry way, you can have a simple tent door, or have the whole wall peel away. Use ZipWall, nothing else is tough enough, I've spent 900 bucks building a room like people would be living in it, and it isn't even as functional as my plastic room framed up with 2x2 studs, which cost me around 200 or less to put up, because I didn't have anything I needed.