Like most 5-door Land Rover Defender that are constructed before 2007 ours also had rotten doors. Where the most common problem with the front doors is corrosion of the bottom part of the frame, the second row doors have to habit to completely disappear.
Replacing a Defender door is easy, but getting your hands on one for cheap isn't. Most 2nd row doors go into the hundreds of euros each. we decided to leave ours in until we could find some that were reasonably priced. At the end of 2016 we got a phonecall from Route58 where we bought the Defender. They had two second row doors that came of a Defender that had crashed.
We bought them for a reasonable price and had them fixed. They were bend to fit the normal profile of the door opening and were painted in the correct color. Big advantage of these newer doors is that they have a galvanized coating to prevent rust.
After the doors were renovated it was time to swap out all the mechanisms, windows and door cards. A task not to underestimate, it took me quite some hours to swap out everything just for one door. Second one went a bit quicker since we new what to do in what order. In both cases we also had to replace the doorhandle after it turned out the bolts were stuck in place.
In the end everything went well and we're set with two new second row doors that are better than the original. And overall they turned out to be quite cheap as well.