In that case I would get a camera in there and see where the leak is, hopefully it's outside. Not sure why you would put a sewer pipe inside the house like that. If it's a real big pipe you can possibly put plastic inside and just dig up the connections.
Are you talking about the drain of the sink? That is usually a 40mm PVC pipe. Just drill a hole through the wall and extend the pipe to a gulley. R10 000 is insane.
If you sand the whole thing it's going to rust before you're even finished. What was the feedback from Duram? I would try a high pressure washer first to see how much paint you can get off.
Is it a wooden frame or aluminium? Wooden Frame you will need to break some plaster around it to get it out. If it is wood rather cut the two sides with a multitool blade at the bottom and remove the bottom piece, if there's water problems underneath it will be rotten anyways.