I've picked up the above vehicle - 209k miles, 1 owner, eye watering service history invoices, mint interior - with insurance bodyshop repaired accident damage. It covers the front bumper rail and right side, front wheel housing, but the repairs definitely weren't to MB standards and the wheel housing has rusted through. It is replaceable, but there is an issue with the indicators not automatically cancelling until you wind the steering wheel about half way around. Ditto, the engagement of the antitheft lock.
The indicator switch was replaced with an MB switch by the first owner.
How can I check that the column is properly aligned when the steering box is set to straight ahead / with or without the steering wheel removed?

With the steering wheel off, there should be a line stamped at the top of the steering shaft, which should be at the exact 12 o'clock position (with the wheels pointed straight ahead), you would see this at the front of the shaft, or face of the shaft, what ever you want to call it. Also there is the tab that cancels the turn sig, this should be I believe at the 9 o'clock position. You will need to take the steering wheel off to see this, or to see the hash mark just take the big bolt off, no need to take off the wheel IIRC.
IIRC it is possible to install the shaft 180 degrees off, but no other way. But of course toe adjustment can mess things up as well. The steering wheel will fit on only one way, there is no variance there due to the airbag and horn connections, but most anything can be screwed up (LOL) due to toe adustment.
Is the wheel pretty close to straight when you go straight down the road?

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Hello Gilly.

The steering wheel points straight ahead going down the road, however if you let go of it the car instantly shoots off left.
The indicator cancel function only works with the wheel almost halfway around to the left.

You have a real alignment problem if the car shoots left if you let go of the wheel, regardless of the position of the shaft. So it needs a date with the alignment rack. Get it to an alignment shop and also mention the shaft is probably installed upside down based on the turn signal not cancelling properly.

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It is the misalignment that bothers me, rather than the pulling left. No, I don't like it, but our previous 300E pulled left regardless. MB couldn't figure it out, even after a total undercarriage rebuild and four, new, matching premium tyres (at our expense).
I think it will have to go to the indy and have the column alignment checked first.