Watch out for the water hardness from the rain barrel. I understand they really get hard water. You could borrow a dKh meter from someone in your club?
Salty, not sure what you mean here. Rain water, being very close to distilled water, is exceptionally soft, except for particulate matter and air pollution it may pick up. THe barrels themselves are plastic, food quality (old pickle barrels are a favorite) and should impart no additional disolved elements to the water.
The primary objective of me using rain water is to blend it in with the tap water to soften what now is a fairly hard blend. I do have a test kit for carbonate hardness, and another type (can't remember off the top of my head). Our tap water is moderately hard, and pH 7.6, fairly high. I'd love to bring it back to 7.0 -7.2 or so with rain water blended in.
Does this make sense, or do you have an issue with my logic?