Stones are fine, they are good for drainage. I would top up with a peat free multiple purpose compost, or mix in soil improver.
Without knowing what way the boarder faces it's hard to recommend plants. It's pictured in shade, but does it get sun at other times of the day?
I would put in a mixture of perennials, including some evergreen ones. In the autumn I would add some spring bulbs.
If you space the perennials out you could add annuals each year in between for variety. Or, if you don't want to do work on it each year, put in more perennials.
A few shrubs can give a bit of structure.