I bought a 10gpd GE RO sink unit from Home Depot...
I figured I wasn't going to bath in it, just coffee, topoff, Kalkwater,and occasional WC...
For the most part I only use two or three gallons a day and "IF" I think ahead I can gather ten or twenty gallons for a WC over two or three days.
I up graded to a four gallon storage/pressure tank from the two gal.it came with.
However, I tend to not plan ahead
two or three days ahead of WC day and some times think a 30 or 60gpd membrane will be my next upgrade.
You may want to look into a quicker/more gpd unit up front, you will find yourself using more than you may first think.