I also looked into UV too Rob, but in the end we went for a water treatment product made in Australia and imported a couple of 25L containers from New Zealand (I'm originally from there), easier said than done with Japanese customs and dealing with the Health and Welfare Ministry, but I always love a challenge.
http://www.davey.com.au/ApplicationandProducts/Acquasafe_Rainwater_Purifier.aspx Not sure if that is readily available in your part of the world.
In the process of setting up water storage for our second home here in western Japan. Don't harvest water from the roof though (yes, a future option if needed), get it from a small mountain stream, or more like a trickle of water unless there is or has been rain recently. It sometimes goes dry, but we do tend to get rain about once a week. Have two x 10,000L polyethylene tanks, one is full of water, connected to a pump and the house. Imported the tanks from New Zealand (in a shipping container) and originally considered selling one but have since decided to keep it and join it up to the other tank. Made another concrete pad for that recently. A work in progress e.g. more piping and also need to make up some seismic restraints too. Enjoying the learning and doing process very much.
Inside the house we only use a simple water filter for the kitchen sink cold water supply. And if we have a group of people staying then we'll often source some filtered water city side for drinks (500 yen for a 4L bottle, with unlimited free refills at the supermarket), where we spend most of the week. So we don't often drink much of the tank water, would do if we stayed there for longer periods.
Not much snow in our area, does sometimes drop below freezing at night during winter. The tanks get quite a lot of sunshine, so even if some snow falls or ice forms it will usually melt in the morning. Having the tanks in a sunny location also helps to reduce the gas water heating bill during summer, I just use the cold water for showers.
Hope that helps. And that your plumber friend comes around, both mind and body, in the future. Also when it rains you will hopefully smile not only for your plants, but your family and wallet 'health' too.
