This is the sensor I had in mind for the low coolant level (blown hose, punctured radiator, etc), www.ebay.com.au Coolant Level Sensor.
I don't see how a LM35, or the factory temp sensor can do the same job as this can?

Opinions seem a bit mixed on weather there's an electrolysis risk. Some say a really low current eliminates the danger, others say you shouldn't use DC at all.