Shortly after I bought the car, it was very difficult to start when warm. The cause turned out to be a failure of the diaphragm (located below the fuel tank) which holds the static fuel pressure.
The engine also made shockingly loud cam noise, especially when driving at highway speeds. I have been told that this is common with this engine as it has mechanical (rather than hydraulic) cam followers - but you should have heard it! It was certainly way beyond normal. Even after adjusting the valves, the noise persisted, so I had the rockers and rocker pivots replaced. This helped immensely, bringing the engine noise to the standard level, but it's still a noisy engine.
When I bought the car, the steering was rather sloppy, so I had the steering box replaced with a rebuilt unit.
The door strikers also needed replacement. This is apparently a very common repair with 123's.
It's a really delightful car to drive. It has a lot of "presence" on the road. The best way to describe it is serenity. It is not a sports sedan, rather it is a compact luxury sedan. It feels very solid (but also a bit heavy!).
Around town it sometimes feels a bit cumbersome, but the motorway is where it really shines: The faster you go, the better it feels.
But make no mistake, it should not be judged by modern standards - the series was introduced over 30 years ago, and even that was an update of the W114/115 series introduced in 1968. So it's not surprising that it feels very charmingly vintage. What a huge quantum leap they made when introducing the W124!
The engine is a big lovely all-alloy twin-cam six. It's got loads of torque. It is only let down by the automatic transmission, which starts out in 2nd gear and doesn't allow the engine to rev fully before shifting to the next gear, even if it is floored. I discovered that for best acceleration, it's a simple matter to hand shift through the gears. Then it really surprises people!
To the smallest detail, all of the parts making up this car are of the finest quality. They apparently engineered their cars with the cost as no object. It's really true that in this case that; "they don't make them like they used to" The seats are definitely the most comfortable I've ever experienced.
It's really an enjoyable classic car.