I'm putting in a half bath and a washing machine in our basement and I'd like to like to connect it to the main stack and drain, which is all 4-inch cast iron. The sweep where the stack meets the sewer line has no clean out and appears to have some corrosion, so I'd like to replace it. I was thinking that I would cut out the sweep about 2 feet up the stack and one foot along the sewer line with a snap cutter and replace it with PVC, making the connection to the cast iron with banded couplings both above and below ground. Above ground the PVC would include a clean out and a connector for the washer drain pipe. Below ground the sweep would include a reducing Y fitting to accept the 3-inch toilet drain pipe. I've figured out the venting and I'm planning to put in a few riser clamps to ensure the cast iron above the PVC is adequately supported.
My questions would be a) Does this seem like a sound plan?, b) Are there any issues with banded couplings under ground?, and c) Is there anything I'm missing?