Sounds like you've got yourself a Kombi, the first clue would be a 3 as the second number of the VIN, the M plate on top of the LH fresh air duct under the dash would answer all the questions for sure.
Bare metal usually attracts some condensation just from the humidity in the air, if you are getting a rainstorm in there you'll have to find out where the leaks are letting in the bonus water, it's likely hiding under a mat or something. Opening a few windows so the inside stays the same temp as the outside will help alot.
Kombi's were basically the stripper model with no interior finishing except a little headliner over the cab area, hardboard panels on the rear side walls or sidewalls and roof were available as an option but not any insulation.
Whatever you do fix all the leaks in the windows, doors and floors before insulating or you'll just trap what you are seeing now and cause more rust.