I run my RO in the garage. I originally bought an Osmotics 100gpd and later upgraded to 200gpd.
Iwould recommend 300gpd as a minimum.
My set up is a long tube to an outside tap fitting, this is coiled up when not in use, preventing freezing etc.
This pipe goes into the prefilters then up to a booster pump, then through a pressure guage and into the RO. It then goes into the DI resin vessel and into the water butt, 250 litre.
It takes me around 4 hrs to produce 250 litres, a bit longer when the resin or filters need changing. I have a time switch on the plug set to 4 hrs so if I forget it the pump will switch off and not run dry.
There is also a float valve in the tank and this shuts off the water if the tank is full.
Barrels take 4 minutes to fill and I set the timer on my mobile to remind me. Twice have had a flooded garage before I did this.
The water butt is raised so I can get the barrels underneath.
All I can say is that the Osmotics system was so easy to install with their instructions and the support via email was brilliant.
It has produed water at 000ppm for a couple of years now and gives me no problems at all.
I screwed batten to the garage wall and mounted it all on this, easy.
In the very cold weather I put a fleece blanket over the unit. I do have a temperature guage in the garage and if it gets near freezing I pop a fan heater on for a while.
Hope this helps.
Chris.