Your first job is to assess how bad the rust is. To be honest it needs welding but it depends how much you love the car or if you can afford to have it done. Our c900 needed a lot of welding so we bought a welder and did it ourselves - including similar door repairs.
The door seal is held on by a large number of plastic clips which push into holes in the door frame and then you hook the holes in the rubber on to them. Pull the rubber off the clips carefully so you don't tear the rubber, and just unhook enough so you can see the bottom of the door - sticky tape the flapping rubber up and out of the way.
The straight rubber section is riveted on, though it's detached on many c900's and might be glued on too. You'll have to drill out the rivets, though you might be lucky and can just pull it out of the way if some are already torn through the rubber.
Then you need a wire brush on a drill or a power file to get rid of as much rust as you can. You can use stuff like Bilt Hamber Deox gel to remove pitted rust or Hydrate 80 to convert any rust that's left.
If the bottom of the door isn't rotten all along, a fibre glass repair would be feasible, though new metal would be better in the long run. You need at get rid of as much rust as you can though or it will just rust through again.
