re: DIYers, how hard is it to replace an exterior door?

It's really not bad at all. If you are handy it should be no problem. Just give yourself plenty of time and have an extra pair of hands available. Like others have mentioned you want to flash and weatherproof the opening for the new door. The door you purchase should include instructions on how to do this for the door. Make sure you buy a couple of packs of wood shims. Place wood shims behind the hinges and remove a screw from each hinge and drive a long screw into the framing.