Work back from the intended use
Water for a garden, washing tools or flushing is a different problem from water for drinking. The intended use determines the treatment, testing and maintenance burden.
First checks
Look at roof material, catchment area, gutter condition, tank size, overflow, frost exposure, access for cleaning and how water will be pumped or drawn.
Caution
Do not imply untreated harvested water is safe to drink. Drinking water guidance needs testing, filtration and local rules handled carefully.