I have a friend that I am going to ask to weld one of these together for me but my back is not what it used to be. How heavy would you say this thing is?

The one I made is right at 11 pounds. You could probably cut a little weight off the handle by giving it a shorter handle and putting a D (horizontal) handle at the top.

The most taxing part of the process isn't so much the weight of the thing as getting the thing into the ground. Wear heavy shoes - sneakers don't cut it...and I wear lug-soled boots. If you keep it sharp, it cuts much easier. Another thing that might make it easier on the feet would be to weld flat bars at the top of the blade...so you're not stepping on the blade itself, but have a wider surface for the soles of your feet.

Black poly will start leaking long before pvc pipe fails.