I need to replace some of the radiator fluid which leaked out (see here), so I thought I might as well do a full flush...

This is the procedure I understand to be correct. Please give me some feedback.
I drive a 1979 240D.

I'll use Zerex G-05 coolant (plan to get 2 gallons) and distilled water for the final filling.

1) Drain the radiator by unscrewing the radiator drain plug underneath it.
2) Close drain plug plus fill the radiator with tap water.
3) Run engine until warm with heater on, then wait 30 minutes.
4) Repeat 1, 2, 3, four more times to rinse system.
5) Finally, replace with a 50/50 mix of Zerex G-05 and DISTILLED water.

My questions are:
1) Is there a block drain on the 240D which I should also plug/unplug during this
procedure?
2) Is there a product I can use to thoroughly cleanse the system (I heard
something about citric acid - but what brand specifically, when and how?) Or
should I use baking soda?
3) How do I properly take care of BURPING (air pockets) - what do I do?

Rino

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1979 240D, W123, 105K miles, stick, white w/ tan interior.