New border - is mixing in bought organic mix/manure a good idea

The borders used to be plastic sheeting with gravel on top so I have removed them and dug the soil over.

I mentioned manure to a guy at the garden centre and he said this was too strong to use but mentioned a pre-bagged product to mix in with the soil that contained 25% fertilizer.

So I am thinking of using that - will it help a lot and how much should I use - should it be mixed in near the top or much deeper to 1 foot?