I think a circular saw with a guide should be fine, maybe a 4-foot level and some clamps. If you're worried about a clean cut, you could get a fine-tooth plywood blade, but I wouldn't worry too much about it. Nobody looks at the bottom of doors anyway.
As far as a guide for setting the knob, most knobsets come with a paper template that you can tape onto the door to get the holes in the right place. If your trying to match a new door with an existing jamb, I would hang the door first so it lines up with the catch before drilling it.
Using the old door as a guide would probably work fine also, especially for the hinges.