Doors & floor
I'm no authority on the subject, but I do believe the floors are interchangeable. If you find some solid floors from a non-wagon donor car, and you find that they vary a little, you should still be able to modify them to work. The differences would be minimal, if any.
I don't know for sure about the rear doors between a wagon and four-door sedan. It would make sense that they would interchange.
The wagons have a specific fuel tank. I have read (on this forum) that the sedan and coupe fuel tanks don't fit the wagons, but I don't know the differences.