But I wonder abou 0 KH... Common saying is that some KH is necessary to keep pH stable. But I'm reading more and more about people who keep 0 KH and it works well.
You need some sort of buffering agent in the water to maintain stability (at least in terms of pH). carbonates are a pretty common one, and tend to buffer towards a higher-pH, but there are substances that can act as buffers for a lower pH (it's been ages since I've had a chem class, so I can't remember any right now...). I'm not certain, but I'd be willing to bet peat, driftwood, and other stuff that adds tannins/humic acids can be a source of a non-carbonate buffering agent.
I agree with what funkman262 said, use the 5-stage RO/DI unit, and add straight tap water to get the gH and kH into a range you are comfortable with, and then add the micronutrients, etc. I'm pretty sure it will be a lot cheaper, less headache, and probably turn out just as well.