Doors are not really hard to find but shipping one cheaply and without damage is almost impossible. Aside from the general condition of the outer skin you might also look for problems with:
One of the known problems with the 944 door (usually seen on the driver's side) is a stress crack that develops just below the window frame. This problem is difficult to repair so look for it first when evaluating a door.
Another consideration is the window channel -they are frequently found to have separated and require a small weld to repair. You may know this is the beginning to be a problem if your window rattles when you close the door - most evident with the window down.
With all of the window hardware out, I got a peek at the window regulator. The power window operation on these cars is often slow. Mostly, that is due to poor electrical grounding or a defective switch. But this lack of lubrication doesn't help. The grease was probably slowly washed away over the years.
The inside of the door is NOT water tight. Quite a bit of water can make its way around the window seal and into the door. This is not a big problem if the plastic shielding is in place and the drain openings at the bottom of the are not clogged. Getting the plastic shield back in place is important.
swapping them out will take awhile due to having to move the window, wiring and door latch stuff. Expect to spend a few hours with this job.
Are you wondering why old cars have that special smell. Well here is one source - water made its way through the plastic shield and over time rotted the door panel. If you're buying used door panels, always check the lower edge for water damage. This one is headed for the trashcan.