Lots of good advice here, mixed with some that's a bit out of date.
First, magnetic ballasts may not be the ultimate in efficiency, but they make up for it in reliability and versatility. Whoever said you can't hang one on the wall was incorrect.
And there's more; it turns out that the 860W CDM lamps that put out such an amazing quality of spectrum (CRI =92!) ONLY run on old school magnetic thouies- set for MH bulbs, of all things. So much for 'magnetics are ready for the scrap heap.'
About them CDM lamps, gets all ya need to know;
http://advancedtechlighting.com/cdmea860.htm
Look at that spectrum graph and tell me that's not a revolution in HID lighting!
On to RDWC; I use it, I love it. Yes, it needs to be chilled, but the same chiller can also chill the rest of your op- and do it for less money than a minisplit ever thought possible. Your minisplit saves on up front cost FOR AC ONLY; don't forget to add more to buy a dehuey and yet more to buy a water chiller- when instead just setting up a bigger one in the first place is much more efficient.
In MY engineering book, efficiency can be used two ways; either use fewer resources for the same results... or get more results from the same resources. I've found that water cooling lets the grower step on the gas in their op a lot more than AC does.
Vertical growing means hood manufacturers are looking over their shoulders. Don't buy them, vertical is stunningly better. If you want to run 2x600W, do it this way; get five foot fencing and install two sections, each 6'3" long, and connect them end to end. This will give you a cylinder five feet tall and four feet in diameter, which is much better for that wattage than just 15" radius.
I use dry nutrient salts for results as good or better than anything from a water bottle. STUPID CHEAP too, especially when compared to H&G, lol
Snaps knows his shit. So do most of these guys. I only contradict their advice when my own experience runs strongly counter to theirs.
Agreed on bennies- aka beneficial bacteria, etc- in your RDWC. Crown feed and foliar spray with it, too. This is not a substitute for chilling your RDWC water to 66f.
Low EC is a good thing. Keep pH in the mid fives for veg; it should only see 6.0 it higher after week 6 of bloom.
I wasn't impressed with The Head Case Farm; I WAS one of their original RDWC/vertical growers- and they threw me out because they got jealous! -and I wouldn't buy their seeds.