Gary, 100 litres, ie 4 x 25 litre barrels will see you through a day.
Okay, the barrels aren't light, but they're not that heavy either; especially after you've filled the backpack for it's first fill.
Basically, what I'm saying is you'll only have a 'heavyish lift' four times in each working day. It's not like you're humping full barrels every twenty minutes throughout the day.
Just partly fill the backpack, enough to clean 6 to 10 windows and it's easy; it takes seconds to do.
Each subsequent fill is easy, obviously, since there's less water left in the barrel.
I'd suggest you start off keeping it as simple as possible, then after a month or two, if you still want to, then look at getting some sort of transfer pump.