Don't worry in the least about using a pump with a metal impeller...Almost every car on the road for the past 90 years has used a metal impeller pump!!!
True, the pump in some instances has been due to the impeller hitting the suction cone, but in my case the plastic one just split in half. I don't believe an automobile is a good application for a plastic water pump, given the high temperature of coolant and cold/hot cycling.
I have worked in engineering in the chemical and refining industries for over 20 years, and have a bunch of experience with centrifugal pumps. You seldom see plastic pumps here; then only due to pumping corrosive liquids and never at high temps. Even these sometimes have a metal core for strength.
