In PenskeParts.com, Engine>Belt Drive, there are two pictures of the belt system. The first (Diagram #11_2190) shows the hydraulic parts, Item 21 is the complete unit,Item 6 is the pulley, and is all you need to replace if the car already has the hydraulic tensioner -- which it should have.

It is confusing because they show the mechanical tensioner as a part used in production, but the Item # is flagged as Not Available (N/A). To add to the confusion, RealOem also shows a diagram of the mechanical unit, but lists the description as hydraulic and gives the same part number as the hydraulic unit. The mechanical tensioner has been used in this location on several different inline sixes, and is discontinued before production of the E53 (X5) began, so I am confused as to why they reference the discontinued part. The X5 is built for other markets, and they might not have power steering in those markets so the parts diagrams that show "without power steering" would be valid, even though there are no cars in North America that appears as the diagram is drawn.