Hey,
Hopefully I'm not intruding on the conversation.
I am not that knowledgeable and think of everything in simple terms.
Since it was not answered before, you CAN dring RO water, but not DI water, as DI water would remove too much of the minerals in water, which your body needs. There are several studies into the effects of DI and distilled water consumption. Basically to sum them all up, ok if by accident, or even for short periods of time but NEVER a good idea long run.
In terms of the remainder of your questions, everyone here seems to have done a great job of answering. You can use a RO/DI as needed, or have it permanently set up with a storage container, pressurized storage, separate tap, or many other methods that suit your needs. My understanding is the only purpose of a pressurized system if to allow the tap to quickly provide a limited quantity of water. As others have said, a 4 gallon system would give you 2 gallons of tap ready RO only(drinkable), or RO/DI water(top off or small water change). Most with a RO or RODI would then have other containers they would fill for their larger water changes run on either float switches or timers.
Remember, 100 G/Day means 4.167 G/Hour or .07 G/Minute (around a 9 oz glass of water) so using all of the pressurized tank would take a half hour to refill. (assuming 2 gallon of storage space) This would work great for top offs, or nano tank changes, but not 120 gallon tank changes.
Again, I'm new. The others who posted have vastly more knowledge and experience.
I'm in the process of setting up for a ro/di myself, so let me know which unit you end up going with.
