There is a Haynes manual that is not bad, but is only readily available in the UK. It covers diesel and gas 124 models. I would imagine that it could be ordered via the internet. I ordered mine from an ad in the Star magazine a few years ago.

If you wish to keep this engine young, I would strongly recommend frequent, hot oil changes using universal grade oil which is especially formulated for diesels. It has additives that deal with soot dispersal, a problem specific to diesels. The most common examples of these oils are: Chevron Delo multigrade, Shell Rotella T, Mobil Delvac and Mobil Delvac One. Any of these are excellent choices, except Rotella T has a higher paraffin content, so if you chose it, you should always drain the oil when the engine is at operating temperature to minimize parafin build up inside the engine.

good luck,