Since you only have an estimated 4' of verticle grow space, I'd suggest using a SCROG, and vegging for at least 2 months per plant. The amount of space you have you can surely get 15 in there of decent size, scrog them, and have a decent harvest.

Problems with DWC vs RDWC when your dealing with multiple plants is the swapout and feeding. I realize that going from DWC to RDWC is a pain in the ass, and I understand that, but when your looking at your growroom that consists of 15 DWC plants, and you have to do water and nute swaps.... well.... I'd rather be in soil at that point.

So, to answer your other questions - If your doing a weekly swap out, just monitor your PPM and add feed as needed. I typically don't, but my PH can fluctuate and cause some plant stress (just lost 2 because of it) as far as water temp, if your doing that many DWC's under those HPS lamps in an enclosed room, if your ambient temp is around your mid 70's, you can expect the same of your water, the warmer your water, the more at risk you are to microbes and bacteria. I like to keep my water around 60-65, this is usually done with a chiller.

Long story short, your grow is too large for a DWC, unless your going to go with totes, in which case you might as well go with an RDWC that way you can do your swapouts via your res.