Drain bolt on m103 w124 is under the 5th or 6th spark plug. You may be able to get to it from above with regular socket and extension between/around exhaust, maybe swivel too. Easy from below with wrench.

Also there is a bleeder bolt on top of engine that will help during refill. New cap will not hurt and check for cracks in expansion tank or just replace it.

I got a couple of pounds of citric acid off of ebay for about 10 bucks delivered; mixed it with about 2.4 gal of water for flush. Distilled water with the G-05.

http://www.w124-zone.com/downloads/MB%20CD/W124/w124CD1/Program/Engine/103/20-0100.pdf

http://www.w124-zone.com/downloads/MB%20CD/W124/w124CD1/Program/Engine/103/20-0160.pdf