What factors should be considered in exterior door replacement?

Before rushing off to the door store, check the hinge screws on the current door. Sometimes just few quick turns of your screwdriver will pull a sagging door back up into alignment. Still have gaps? Maybe you just need to replace the weather stripping. But if the frame of the door is out of square, this would suggest that it’s not the door slab that’s moving out of alignment, but actually the surrounding walls that might be settling. Could it be that the water that caused that rotted sill 25 years ago has been working into the walls? Check for rot at the bases of the walls and the door. You may have to replace the jack studs or sole plate on either side of the door when installing the new door. The steel frame residential door you’re thinking of is designed for security rather than keeping the door aligned. But I think you’re on the right track to be thinking about “something else” than just the door itself.