Loc nar's info is right on...if you have well water yer almost definately gonna need the RO don't get the ones that have the DI cause that's mainly for drinking and taste and you want the water to have as close to 0ppm as possible to start

GH makes good nutes so yer fine there

How large is your res? How does it empty?(hopefully it has a bottom drain valve) I have the aero9 which has a 20 gallon and found that less than 10 led to ph and ppm flux so just keep it topped off and you'll be fine...the only time I let the res volume drop is the few days before a change cause there's no valve and 1 gal = 8 lbs haha 160lbs in a swuishy open container is not fun to move...I let it get just under 10 before I'm willing to attempt the strongman event ;-)

Post any questions ya come up with I'm glad to help and good luck!