As above, first determine your tds level of the water. Also if you do need to go down the route of a RO, yes you can store that in the garagr but it would be worth insulating it come winter months. But its also advisable to look on ebay for a water pressure gauge as the water supply from flat to the garage may want a booster pump, which are easily sourced on ebay as well and relatively cheap.
Storing the ibc tank outside, yes you can. I have mine in the garden just outside my rear window up against my house. However i opted for a black ibc which will prevent water going green. Ive slanted some boards on top for a run off for the rain etc and boards at the side for a slight insulation again. Ive not experienced a bad winter (only done this method 1yr n half with ibc tank), but even then it would only be top inch at most freezing. If had no protection, which means your starting with really cold water on a really cold day, so could possibly freeze quicker in hoses etc.
But first sort out water tds and then decide which method? Trolley or van as in your other post. A long with how much water do you think you would need.
I sometimes use a trolley, but because i have a van system i dont worry about reserving water. So using a gardner backpack on a trolley, cleaning a full house (average 3bed house) could last 2.5 houses.
If a full house plus conservatory, then not quite that plus 3/4 of next house.
If upstairs only and reserving your water you could get 4 - 5 average size house cleaned per full. To give a slight idea.
Most would use 350ltr in a van system, but many and myself included have opted for 500ltrs which means you may not run out of water in a working day. But thats dependant on the payload of your vehicle.
So you can see theres alot to take into account. But we're all here to help and can do so better once you decide on a course of action