DieselAddict,

The drain plug on a 240D (W123) is on the block next to the starter. I believe the 300D is the same. It is not hard to get to or see, but I would disconnect the battery when you do the job. I can be kind of clumsy and when operating in confined spaces this characteristic can become accentuated. I have managed to nearly weld the wrench to the starter on its way to the drain bolt in the past, which is spectacular enough to make the pain of skinning your knuckles just before the sparks started seem like it happened a year ago.

The bolt has a Copper or Aluminum washer under it and when the bolt comes out, sometimes the washer stays on the bolt, and sometimes it doesn't. When it doesn't it follows the stream of coolant coming out, and can be hard to retrieve. Just be aware there is one there (unless the last guy lost it) and you should have no real difficulty.

I know of no filter in the sytem, and have never noted any discussion of one in any manuals. Good Luck, Jim

__________________
Own:
1986 Euro 190E 2.3-16 (291,000 miles),
1998 E300D TurboDiesel, 231,000 miles -purchased with 45,000,
1988 300E 5-speed 252,000 miles,
1983 240D 4-speed, purchased w/136,000, now with 222,000 miles.
2009 ML320CDI Bluetec, 89,000 miles

Owned:
1971 220D (250,000 miles plus, sold to father-in-law),
1975 240D (245,000 miles - died of body rot),
1991 350SD (176,560 miles, weakest Benz I have owned),
1999 C230 Sport (45,400 miles),
1982 240D (321,000 miles, put to sleep)