I would agree with the 50 because I happen to own one. But you don't need a 50 to do depth of field work. My primary lens is a Canon 28-200 and it's giving me shots like this in bright sun light.
The point is if you know how to use the lens you have you won't need a new one. But, if you decide on a new one the 50 f/1.8 is pretty good.
50 ml fl
No you shouldn't need to buy. It wouldn't hurt to experiment with the sigmas if they fit your camera.