Many people have asked about the depths of the beds. The rules are pretty simple.
The reservoir zone should always be less than the maximum wicking height of water in that medium. If you make it deeper then the water will never be pulled out of the bed and you will be left with a stinking mess in the bottom of reservoir. Generally a good depth is 300mm but it can be less, in which case you will simply have to water more frequently.
The depth of the upper soil bed depends on the root depth of the plants, obviously deeper rooted plants will grow successfully with a deeper bed, but may required overhead watering for a bit longer while the roots gets established. For vegetables a depth of between 200 to 300mm is common. The great thing about vegetables and other plants is that for the most part the use of heavy machinery is not needed.
If you make this upper bed to shallow then the surface may be wet which will encourage fungi and moulds in some plants.
If you want to use a high bed to save stooping then you should fill the bottom of your container with some suitable material to raise the base of the bed so the bed itself is not too deep. Any fill will do as long as it is not sharp and will cut the plastics.
If you have a waterproof container, and I see that iron containers, like water tanks, are becoming very popular then you can simply place a stick down the side of the bin so there is a good drain for the excess water to escape. Good drainage of the top soil layer is critical for making wicking beds successful.
If you are using an open wicking bed, as used of fruit trees or deep rooted plants, when the water wicks up, then sideways and downwards outside the bed, then keep the lower layer at about 300mm (or less) and leave about 100mm for the water to wick up and over the side.