The workshop door was in some very seriously sorry shape: 
As could have been predicted, a hollow-core unfinished wooden door can’t really handle being an outside door. Fortunately, I was able to get a solid wood door at the Habitat Re-Store for $5, and it even has a nice window to let more natural light (and views) in.
The new door was half an inch taller than the old one. I could have trimmed the door, but it seemed easier to pop off the threshold and cut it down a bit. 
The biggest trick — not that it was much of a trick — was mortising out the places the hinges would rest. I used the old door as reference; there were a few spots where the old hinge area came a bit closer than ideal to the new one, but it seemed to work out OK.
And voila – new door, much nicer than the old door. $5 for the door, $7 for some shiny new hinges, and an hour or so of work. 
Now I’ve got some cleaning to do both inside and out, but that’s a topic for another post. 😉
Am I right that your new-to-you door is being held together with steel straps?
Yeah, there are some reinforcing. $5.