If you buy a bottle of DI water to put in your battery/steam iron etc, it says on it "Do not drink"
The water we produce is the same stuff. Incidentally, tropical fish afficionados buy RO water from pet shops at vast expense - possible earner?
The reason you shouldn't drink it is that pure water has an aggressive need to revert to its natural impure state - as soon as it comes into contact with any substance it starts dissolving material, such is the nature of naturally occurring water.
If you drink it, what's the first soluble substance it comes into contact with? That's right - YOU - or rather your mouth/throat/gullet/stomach.
Hence Sparklebright's guts ache 
Drink enough and it can seriously damage your stomach lining
I've heard many others say this, but R/O water is about the same as rain water in purity, mine after the R/O is 0.002 ppm, rain water is around 0.006 ppm.
As soon as it hits your mouth it is contaminated and is no longer pure, by the time it hits your stomach it will have mixed with the food and drink you have consumed along with your digestive enzymes and stomach acid!
And as said, R/O is used all around the world to purify the likes of sea water for human consumption, the ones we use - such as the small units by RO man - are mostly used in the food industry.
Not sure about D/I water, I know the resin itself is poisonous but no idea if it is transferred in any way to the product water, I've yet to see any definitive, independent non-biased research on the subject.
Regards water production rates...don't panic if it only seems like a dribble being produced!!
I used the small RO man type unit for over 4 years and even at those low rates of production it was enough to keep 3 window cleaners going with WFP.
The Merlin will produce water much quicker than the small system I had, although it isn't as efficient in general due to it's faster production rate, although that also depends on the ppm of your tap water.
I didn't read in your post (or didn't notice!) what your mains pressure was?? Mine is around 65psi and anything around that pressure will keep you supplied with enough water with no problem at all.
Ian