SL is right, If you elect to keep the heater working(I do, and it has not been bypassed in any of my cars), then keep it upto date. Change all the hoses(on my S class. I had about 14 hoses, and I changed them all!!). Remember use distilled water only!! While your your replacing your coolant. Change the "radiator cap" and change the thermostat, so you car gets better fuel mileage..

Best of luck,

Martin