If your internal carotid arteries are normal wouldnt the external ones be normal as well?

No certainty & be. aware that only the opening is usually evaluated by ultrasound or angiography. While relevant for blood flow (at the moment), atherosclerosis is in the walls, does not produce narrowing & plaque ruptures leading to clot promoting debris shedding into the blood stream & clots are what creates strokes, not narrowing. Study: http://goo.gl/aRhOU2, the image & my answers on healthtap for understanding.
Probably. The common carotid artery divides into internal and external branches. The internal carotid is much more important as it is the major blood supply to the brain. When atherosclerosis develops it often starts at the branch point (the "bifurcation.")