On your SLK I think the coolant service interval is every three years. Starting in '03 the interval went to every 15 years or 150K miles. make sure you use a proper coolant. The typical green stuff is not approved. Measure the amount you drain out and compare that to the capacity and you can see if you've gotten most of it out. Flush it a few times with water and then a 50-50 mix of distilled water and the correct coolant.
I recall changing the coolant on a Caterham 7 I used to own.
In order to ensure that the air was expelled properly, Caterham recommended raising the top of the radiator so that it was the highest point in the cooling system - while the engine was running to make the water pump work.
We had to use an engine hoist to lift the front of the car sufficiently.
Oh, how we laughed at what fun the manufacturer had made of maintaining it!