I am not sure what sizes are the Line Nuts and the Back up wrenches You would need.
I also don't have a W115 240D.

When I did that sort of work on the 300D I used a Crows Foot on an extension and I thinned the Head on a cheap Wrench to act as a backup Wrench.

I don't know if the Cut Down Combination Wrench sold by Mercedes Source would be helpful in Your case or not.

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84 300D, 82 Volvo 244Gl Diesel