Hi Martin, that is looking really good!
I#d say model all of tehm, as they are not too difficult and still a great practice, but (and is a general rule) if you can get away with copying and scaling and rotating, without it being noticeable, you can do that (woudl deifinitely do that with some trees ;)).
The bottom can be left un sculpted if you want, but if you want to copy stones, it can be an advantage if you can turn them over for a different look. The backside (from camera view) should look good, as the camera will move around.
And James, I’d say everybody makes all stones.
As to how close to the artwork, well it should definitely match the style as close as possible, but the shape and details do not have to match exactly (I’d try to stay close to the size though). Like, for the trees, not every leaf has to be exact like in the artwork.
