Just a few questions...

My experience with overcrowding is that you'll more than likely yield more if you grow less and train more. In that tent i would do 2 plants. Train them if you want bigger yield.
400w is probably perfect, wont hurt to go higher if you have the ventilation ability. If your properly venting, then you could buy a dimmable ballast and run it at 600w.
DWC is great but can be tricky. You can get the same yield by growing in a coco hempy bucket. Very simple setup and much easier to maintain than DWC. Just need COCO bricks and perlite and 5 gallon buckets.
If you go with DWC just build it yourself. If you feel comfortable with a RDWC set up, then build that because it's going to be more effective than a single bucket DWC setup. BUT, like I said i would HIGHLY recommend switching your plans to a Coco Hempy Bucket.
 
Make sure you plan on getting an inline fan and carbon filter for your closet unless you somehow can vent outdoors.

 

^^ I agree, 2 plants would be the max in that tent apart from a sea of green (SOG) grow IMO.. I too wanted to start with dwc, but was recommended a hempy bucket due to my lack of supplies, and I'm glad I did it! It gave me great experience with mixing, my personal water chemistry, environmental changes during flowering etc.. All the while not having to worry about fluctuating ph (AS MUCH), and water temps/dissolved o2 levels etc as with dwc.. I found it as an excellent primer, and the yields weren't to say the least.

I went with 100% perlite with my hempys as I found it easier and more predictable than what I had read about coco.. I first went 80/20 perlite vermiculite, but then went to the perlite only as I found it to be much easier to adjust ph.. You'd think that perlite couldn't hold much water but it wicks very well and I easily went 3-4 days between feedings until later in flower.. If you need some more direction just let me know.. Good luck

 

400 watts is absolutely not overkill. I'm running a 400 watt dimmable MH/HPS in a 3x3 tent. I went from seed and put them straight under the MH when they popped out the rockwool. Granted, I dimmed it to 200 watts at first and progressively bumped it up until I got to 400. Agree with the others about the number of plants. Personally, I would only do one in there, put a big scrog on her and let her go. I've got one DWC bucket in my tent and she's about to bust out the sides. Granted, I made the mistake of putting the screen on about a week or two too late. I'm a big advocate of single DWC buckets. I do admit, however, that I am still a novice at DWC (on my first DWC grow) but everything has gone great. PH swings have been almost non-existent. Maybe I have good water or maybe it's just I'm so OCD and I stay on top of my shit. And even though I have no experience with RDWC, I still prefer single DWC buckets (unless I end up getting a whole ROOM to grow in! Ahhh maybe one day!). Mainly because I have more individual control over each plant. ESPECIALLY if I am running different strains. Different strains might take different nute concentrations. One plant might need more calcium than another. The list goes on and on. Again, I have no RDWC experience. But based on what I DO know, I still prefer the single DWC buckets.