OK, I've found my answers, unfortunately, I have to say, from sources outside this forum. I am posting them here, for the benefit of other forum members who might come across this thread looking for the same answers as I did.

I'll quickly open and close a personal note. I have to say that I have been a member of this forum for over three years and during this time I had always received prompt and extremely helpful answers to all my questions. This time around (after about a one-year absence) is the first time ever that I have encountered so much reticence and even that overtly antagonistic reply (in very poor taste) from the last poster... Perhaps, this time around, the good ones were busy with other threads and did not notice this one, or maybe are away enjoying their summer vacation somewhere or something. Regardless, I am very much grateful to this forum for all the help I had received in the past and am always willing to contribute in return to it as much as my knowledge/experience allows me to. End of personal note.

So, to the answers. Citric acid can be bought locally from any restaurant supplies store (food or lab grade doesn't matter - any powder citric acid will do the job). I just located one in my area which is selling me a 7-lb. container for about $14 (at their wholesale price!). The amount of dishwashing detergent to be used (in my case, I am using Dawn) is the whole container (16 to 20 liquid ounces). Make sure to never pour the citric acid or the dishwashing detergent in the radiator directly without first mixing them well with plenty of HOT water!

Follows a set of instructions from another forum which I am attaching verbatim, one among several which I have found very helpful and highly informative. Enjoy, and I hope this helps answer some of the questions from first-timers at this procedure such as myself.

How to Flush the Cooling System with Citric Acid

To start with acquire 2.5 lbs of Citric acid. Food or lab grade doesn't matter. This can be acquired at many food suppliers locally, online or ebay.

Now is a good time to locate your block mounted drain plug which will make each step of this far quicker.

1. You will need to drain and flush your cooling system to remove as much anti-freeze as possible before you start. You should flush at least three times with clear water. Running the car to operating temp each time and allowing it to cool enough to comfortably hold the lower radiator hose or the bottom of the radiator between flushes. Make sure your heater is set to run full blast (maximum temp setting) during each and every stage so the heater core is cleaned as well as the radiator, and the engine block.

2. You need a degreaser, preferably non-sudsing one. Shout will work as will some dishwasher detergents (properly dissolved in hot water completely first) or get Some Prestone radiator flush from the parts store. This will degrease however is too wimpy to do much more than that. How much is enough? I would use a 17 - 20 once container. Better to err or the side of too much than too little and not degrease the system enough. This is a critical step as the Citric acid will not do its job with any oil or lubricants from the antifreeze in the system. Since you are taking the time and expense to this then it would be best to do it properly. Mix the degreasing agent of your choice and fill the radiator full of water, and run the car at least 20 minutes after it is up to operating temp. It is easiest to do this with a short drive. Allow to cool down and flush several times until the water no longer feels slippery that you are draining. Three times should be enough.

3. Mix your citric acid (it’s a dry powder) with 1.5 gallons of hot tap water in a clean bucket or other container. When it’s been completely dissolved pour the mixture into your radiator and top off with tap water.

Take a drive....of about 30 to 60 minutes duration (with heater on maximum temp) after it is up to operating temperatures, you need to get a good vigorous circulation so idling in the driveway will not cut it.

I would avoid leaving this in the radiator for several days or even over night so try to do this step when you have the time to complete it, and the first couple following flushes within a few hours at most.

When the engine has cooled down drain it well, and refill with tap water run up to operating temp for a few minutes then drain when its has cooled off. You do really want to drain with the block plug removed as well as the radiator drain or removing the lower hose. Sediment or rust flakes will not exit the system via the radiator and trust me there will be a lot. You want to do this at least 3 times. This is very important. If you wish you could also do the next step and dissolve 1 lb or baking soda (sodium Bicarbonate) into a gallon of hot water and use this on your third flush. This will help neutralize any pockets of acid that may remain. If you do this step you still must flush at least 3 more times thoroughly.

4. Refill your cooling system with a 50/50 mix of antifreeze that is recommended for your vehicle and distilled water.

This is time consuming and will take a weekend to complete....but you will only have to do this once...and it will be a better more thorough flush than any shop will achieve.

NOTE: As far as the fluids being drained, you need only be concerned about the initial draining of coolant. Please capture the initial drain and take it to an appropriate disposal center. All subsequent flushes are almost 100% water, water with a biodegradable degreaser, or water and citric acid and are safe to be drained straight to the ground.

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