Better: take one of the existing engines, sort the moves from the best to the worst, if the top move has score S, randomly choose among the moves with score >=

Better: take one of the existing engines, sort the moves from the best to the worst, if the top move has score S, randomly choose among the moves with score >= 0.9*S. You can simulate a better/worse player by increasing/decreasing the factor: 1 plays as well as the chosen engine can do, 0 is typing random (yet valid) moves on the keyboard.