1) According to my Haynes manual the car should take 11 L of coolant. I would use Zerex G-05 as it is much less expensive than the Mercedes coolant

2) I would drain the radiator using the drain plug on the bottom. If you are just replacing the T-stat and the coolant looks good you can just collect it in a clean container and reuse it. If it looks scummy don't forget to drain the block using the drain plug above the starter and the heater core.

3) can't help there...I don't have an expansion tank and always fill through the radiator cap.

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1979 240D- 316K miles - VGT Turbo, Intercooler, Stick Shift, Many Other Mods - Daily Driver

1982 300SD - 232K miles - Wife's Daily Driver

1986 560SL - Wife's red speed machine