Good luck. Having drilled and tapped this hole myself (but with the unit off the car and stripped), the problems are;
1). Creating a smooth surface on the PTU boss face for the sealing washer to bed against.
2). Drilling the hole perpendicular to the smooth face
3). Getting the swarf and tapping cuttings out.

See my earlier posts with internal photos of my stripped PTU - even after drilling this hole, you will not get all the oil out because there 3 compartments at the bottom of the unit. From experience, I found I could suck out (using a soft tube and compressor driven suction/brake bleeder) nearly as much as can be drained out. I now have a different Freelander and would not bother to drill this drain hole.