One barrel may not be enough to water the whole patch if we get a dry summer. You could mount a second one on a trailer and collect the water from your roofs at home, then trundle it along to your veg patch.
We use those giant 1000l black orange juice barrels, about £160 now I think, to collect water from various buildings around the place. Some are for livestock water, others for irrigation. We have had them freeze solid in the winter, but being rubber they don't split. They occasionally overflow so we run a hose to the ponds, to flush them through.
We don't have mains water. Where the water has to be moved uphill we use a small pump, but mostly the system is designed to work with gravity.
I like the idea of making sloping roofs without a shed underneath to catch the rain