You should get a handheld
TDS meter and measure your output water.
You shouldn't have a problem upping the output of the unit, provided the
RO chamber will hold a larger membrane, you would do better to address this to one of
RO selling sponsors in their forum, they should be able to guide you as to what is safe to upgrade to. Personally I would go with about a 75 gpd unit. As for the
DI, pretty much any
DI cartridge will do, if you have a TSDS meter, if not, make sure you get one with a color changing resin so you know when to change it