Currently there are 40 native Standard Zoom lenses for Canon 50D and all of these lenses are autofocus (AF) type.
Canon 50D doesn't have a sensor based image stabilization system but 22 of these Standard Zoom lenses features optical image stabilization.
Since Canon 50D has environmental sealings (Weather Sealing), you may like to couple your camera with one of these 9 Standard Zoom lenses with Weather Sealings.