Essentially, CAN Ahmed become a very good defender, or is there something that limits him there?

Physically, he certainly has the speed to be one, and the length. I reference what I think was a point in a CC article regarding Justin Robinson as the Point Guard. Coleman said "if you're going to play PG for this team, you have to be able to keep up with Chris Clarke and Ahmed Hill". Those guys get up the floor quickly, so the speed must be there.

Does he lack 'quickness' somehow?

Or will it simply come down to 'want to' and he'll need to put out more effort on the defensive end? Because with Bibbs and Wilson graduating, I can see him getting a lot of minutes back next season if he does step up the defensive side of his game.