Hi Chris,
This is the link to the R/O water company I purchased my small portable r/o filter from.
http://www.premium-water-filters.com/reverse-osmosis-filters.htm#FMRO5J
the one I had was this one
Watts 5 Stage Reverse Osmosis Water Filter System
50 GPD with Standard Cartridges.
The company is really great to work with. The reason i picked this one is because It actually came with a number of different sized metal heads that screw directly into your kitchen or bathroom faucet so you didnt need to do any frustrating plumbing shit under the sink to hook it up.. Because it does go through your tap it will take several hours to fill your rain bucket up, so dont leave it running if your not going to be there for the first time until you can determine how long it takes to fill. They send you long white tubing so you can leave the r/o unit on your kitchen sink and run the small hosing all the way into the other room where it will drip slowly into your rain barrell. below...
purchased from Walmart, and was great!
i would stick the white tubing from the r/o unit right into the grill .
One other trick. Place this barrell up on a few crates. That way its easier to pull the water out from the hose. Your water will be zero ppm and if you use nutrients with ph buffers all you need is your nutes. I used
General Hydroponics FLora Nova Bloom for flower and veg in coco and hydroponics grows.. . it has ph buffers.