This is OM603 from 300D not an OM606 from an E300D as you mention. What year is this car and what are the first 6 digits of the chassis number?
quite good engine, but has an issue with the head casting in the earliest models. Lets start with the basics - are the glow plugs working?, are your injector pressures correct.
I see that the injector lines are missing the cushion pads - this will cause the lines to fracture, they are very expensive too. The pads and clips are not that pricey, I think all of them cost less than 20 euros at best.
In another picture you show the engine with oil cap off - what are you showing here? are you trying to show us what mechanic showed to you? - If you are then your mechanic should not work on this engine.
As I said - under what circumstances does your engine start to smoke and is this smoke from the tailpipe? or the oil cap?
the car will puff some black/gray smoke on startup - its very normal for all diesel engines to do that. Does the engine have starting issues?
who timed the pump on this engine?
also mention when was the last time the fuel filters were changed and if you have ever cleaned the intank filter
does this engine have its correct thermostat valve installed?
is it hard to start the engine?