Hi Brad,
That looks like a good job. The fact that the bottom is open means you should have good drainage, but I would do as you have suggested and add a good deep covering of clean hardcore/gravel to ensure that water being held in the planter to excess. A good foot (30cm) of growing medium would be enough for most garden plants. Topsoil with added sharp sand mixed with a good multi-purpose compost would be my mix and will get you off to a good start. At least 60% topsoil with 20% sharp sand and 20% compost should see you right.
You can use sharp sand (not building sand, which is too fine) from a builders merchant, it comes from the same hole in the ground that horticultural sand does, but is a fraction of the price. There is no salt in it as is sometimes mistakenly suggested.
I hope this helps,
Ralph.