I'm in the preventative water pump replacement camp (about every 65-70k miles), I changed mine over the summer along with updating to the low temp thermostat. Hopefully, you won't encounter too many issues from the lost impeller bits, several people have continued to drive their cars with impeller bits in the system. They most likely get stuck somewhere in the radiators and oil cooler. It's not ideal, but it probably won't self destruct the engine too quickly.
Any idea what caused the rust?
I've never seen rust like that on an internal component, maybe someone was topping off with tap water?