Car Owners Hub UK - Built by Drivers, for Drivers

A 520d is on my radar as a potential replacement for my existing cab. Which engine requires less maintenance and is more dependable? Every mechanical task is completed by myself.

If you want advice on which 20d engine to use, the members will be able to help you out if you tell them the year and model designation or provide the engine code.

The year 2023 is in my sights.

I would guess a later model car, maybe with a B47 engine, as it will be a taxi. You should replace the oil more frequently than suggested, particularly if you do a lot of short excursions in your cab, because these are more dependable than N47. Perhaps every 7k isn't such a terrible gap after all.

Additionally, do you ever go for a nice run to make DPF happy in your day-to-day?

In my opinion, the B47 is the best engine to choose based on my study.
When it comes to upkeep, I replace the oil every 5,000 miles and make sure to have good runs to get the dpf hot. If I were to get 20,000 miles out of the car, my main concern would be that it will likely just have had one oil change. The engine life of the vehicle will likely be reduced.

And which gearbox is the most dependable?

Twenty thousand isn't a huge deal if it's just had one oil change; what worries me is the accumulation of sludge that occurs as the mileage increases to twenty thousand every oil change.

If you're riding on an automatic, the ZF8 is a dependable gearbox. But, the filled-for-life claims are complete bullshit; according to ZF, the optimal period for servicing is between 50 and 75 thousand km.
50k for vehicles that are driven more in urban areas and 75k for vehicles that cover a lot of highway miles

For the most part, I'd prefer to keep to the 40k intervals that Mercedes recommends.