if you want to open a valve and get out a gallon or two without waiting, or supply an ice maker etc then you will want a system with a bladder/captive air tank.
If that is the case, to get the efficiency you want, it will need to include a permeate pump.
if that is the case, it will not be portable
if the on demand is not an issue, you can use an aquarium style setup, where you open a valve and fill up a bucket or some type of reservoir
captive air tank with permeate pump.... 200-250ish
aquarium version with no tank or PP...... 125-150ish
I would get a 100gpd either way, usually cheaper down the road since they are the most popular membrane size