Hi ML,
Gosh, that seems like a lot of work and I wonder if the amount of rain falling into the gutter and draining into a container will be worth the trouble and expense. You plan to sink it too so that water from the garden also drains into it? I would think that they will be clogged with soil in no time.
There has been a thread on this subject before, something about water catchment, maybe the search function can help you find it.
If water is a problem on your lottie, remember that most crops hardly need watering after they are established. If you spoil your crops, their roots will not go deep to find water. Don't let the plants let you do all the work, make them work for water themselves. Beans for example, once they are growing, I only give them some water when it's very dry during the flowering stage. Try to find out which crop really depends on you for water.
Good luck with your lotty.