Diesel_Bomber's suggestions ...
Some good points here submitted by Diesel_Bomber:
I'm no expert, but I do know that the average 55 gallon drum won't handle city water pressure. It may be only 40-80psi, but there are a LOT of square inches on that barrel, and that adds up to several tons of force in short order. The pressure tank on my well is constructed a lot more like a propane tank than a 55 gallon drum Underneath the insulation, so is a water heater's tank.
Instead of a 55 gallon drum, I would look at a pressure tank similar to what is used for wells, or perhaps a second hand water heater that has a tank in good condition.
As an added bonus, the water-storage tank could be painted black and put outside in an enclosed box, with the side pointed towards the sun being glass. You'd get your water storage, and a bit of a head start for your water heater.