The regular bi-metal blades in the sawzall will cut it but if there is any good iron there each cut will take a couple blades and about 20 minutes. I would go pay $25 to rent the snap cutter, and if it crushes the pipe only then would I use the saw.
Donuts are for fitting a pipe into a hub. You won't have any hubs left, so you want to use banded no-hub couplings.
If the sewer is working, the only liquid left in the pipe will be that that is left in the sludge. Lay plastic sheeting down under your work area and catch running water as best as you can with buckets. If using the saw, it needs to stay above the level of the bottom of the pipe or water can run into the saw.
The old cast iron has not been cleaned and prepped for the last 90 years, so I doubt it will help much to do it now.
Make sure you support the new PVC pipe at max 4' intervals to prevent bellies in the pipe.