The attachment you originally posted, do not buy. It will not install the post sturdy enough. I am in the process of opening a construction company piggy backed off my landscape company and my main focus is going to be installing mile long fences for ranches and farms.
I have been researching for a couple of years on the equipment I will be purchasing and the pounder will be:
Kiwi skid steer post driver
The Kiwi driver allows you still install plumb even if the ground isn't level or allow you to install your corner post on the angle that they are to installed at for high tension fence. They sell it at Kencove and I highly suggesting it. I can go into more detail if you want through Pam's if your interested.