First of all you need to check about Laws regarding a water pump in your flat, if its illegal and how vigilant is the management against such stuff and watch out for nosy neighbors
Secondly i think its easy to install.. buy plastic tank install it with pump and a cut off valve behind the motor. Open the valve start motor fill the tank and close the valve.
when the valve is closed no water will come in the system or go out. since you have 3 hrs of water ideal time to fill it during that