A word of caution, 300gpd refers to AMERICAN gallons which are different from European gallons.
1 US Gallon = 3.785 litres
1 UK Gallon = 4.55 Litres
so a 300 GPD unit will produce around 1135 Litres per day.
But this is dependent on your tap water pressure. If, for example, you have tap pressure greater than 60 psi, you will get greater production. If less than 40 psi, much less water will be produced. (you can of course add a booster pump)
Also, becase I live in a flat with stairs I need to be able to use the 25 litre containers to carry water down to the car
I would see if you can get a bulk discount with your osteopath, because that sound like hard work! I don't envy you.
I have a suggestion for you that might save your spine:
If your local tap water is not too hard (or even if it is), you could consider a DI only system. These systems use only resin to produce water, and are very fast to fill up. You could load your car with say 6 empty cannisters and use a customers outdoor tap to fill them with pure water. This would take about 5 minutes to fill those cannisters.
The advantage is that you cold fill up anywhere there is an outdoor tap. If you dont like the idea of using other peoples water, you could stop at a petrol station on the way and fill up using their water.
I rather do that than be carting heavy cannisters down stairs every day. Also, there is a limit to how many you will fit in a punto, so you'll have to nip home a couple of times a day and do more of it. With DI you can refill on-site.
DI systems are usually cheaper to buy, but a bit more expensive to run though. This depends on your tap water TDS
-Philip