Re: Engine coolant
If by empty you mean the header tank is empty, then topping up with water is fine, even running with water only if fine but only in the very short term, coolant additive is a corrosion inhibiter as well as an antifreeze, I wouldnt advocate mixing additves either,
you will need your coolant tested for the ratio of additive/water.
drain it all and refill with the correct amount is the best solution to all of this,
..where has your coolant gone, you need to sort this out first before wasting money on refilling the system.