How Deep To Dig Out A Raised Bed?

macman

I'm just finishing a raised brick bed (well really more of a slight raised edging, only 1 brick high visible) in the front garden, which will be surrounded by slate chippings. The edge of the square will then be planted with box trees (about 20cm tall) to make an ornamental design.
The soil is extremely dry/dusty and poor due to a large nearby tree which removes any available moisture from the soil. How deep should I dig out within the bed-I've been told about 8 ins should be enough?
Should I then line the bed with some plastic sheeting and drain holes to retain more moisture, or will this waterlog it? Plus some small stones?
Finally, what soil mix should I put back in? I'm guessing a blend of topsoil and compost/manure, but what proportions please?
It's not a big area, less than 2 sq m.

mrbadexample

I dug mine down about 20", until I began to hit subsoil. Only because it used to be turf, so I wanted to bury the turf at the bottom of the bed to get rid of it and avoid losing volume. I don't think I'll ever dig that deep again - just a spin over with the tiller once a year.