Small tank in boot of car (125litres), 60 meters microbore, 18ft pole (to start with), 100gpd Ro-unit (if on good water pressure) at home pumping into 2x 210litre water butts with submersible pump(or second-hand 1000litre IBC if you have the space).
This system is cheap to set up, is adaptable (you may think that you will only use it on upstairs but within a year you will be using it on a lot, lot more), you can add bigger tanks, more poles, hoses, etc and even increase your water output.
The above is easiest option if you are looking at waterfed pole work for more than 2 years,
...................but everyone will have their own idea, which is why ultimately you have to decide what will work for you, it is your desicion, everybody's set-up is slightly different.
Alex