You want to drain the block as well as the radiator. Be sure to set the heater to max heat, to allow the heater core to drain. You will probably want to do the citric flush. The MB stuff is recommended, but others have posted some off the shelf citric acid cleansers. The citric acid will help in removing the scales and crust from within the system.
After flushing you'll want to rinse and drain the system probably three or more times. This can be a pain on your car, as you have to fill the system through the expansion tank, I think. It may be a pain, but you'll have to do it.
Not sure exactly where your block drains are on the 103, on the 117 they are near the motor mounts (left side in front of/right side behind the mount) and should take a 17 or 19mm socket. Let the car cool a bit before taking out the block drains or you may get scalded.
You shouldn't need to degrease the system if you haven't had a head gasket leak (another flushing using degreaser).
It couldn't hurt to replace the T-stat while doing the coolant change. Be sure to index it correctly.
For refilling, you will find it easier to pre-mix your coolant and water before filling the system. Shoot for about a 60/40 mix and you should be fine. Might want to add a bottle of water wetter to the mix.
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'02 CLK320 Cabriolet - wifey's mid-life crisis
2012 VW Jetta Sportwagon TDI...at least its a diesel
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