The brief
The C180 had got hard to start, the cranking slow, sometimes just a click instead of a turn-over, and a couple of times it hadn't started at all on the first try, with a noise from the starter when it did engage. The car was past 180,000 km, which is the kind of mileage where a starter motor reaches the end of its life. The starter motor is the electric motor that spins the engine over to get it running. It has brushes, a solenoid and gears that wear with use, and when it's worn it cranks slowly, can stick and just click instead of engaging, and gets noisy. A starter doing all of that won't get better, and the next step is the morning it doesn't start at all, so it needs changing rather than chancing it.
The diagnosis
A test confirmed it: the battery and the charging side were fine, but the starter motor was drawing high current, cranking slowly, and the solenoid was sticking, which was the clicking and the intermittent no-starts. So it was the starter at fault, not the battery or the wiring. That's a replacement. A worn starter is a sealed assembly that's done its miles, you don't rebuild it on a car like this, so it was getting changed.
The work
The old starter motor was disconnected and unbolted, and a new genuine Mercedes-spec starter fitted in its place, the wiring reconnected and torqued, the connections cleaned. Then the engine was cranked and started several times to confirm it spun over quickly and caught cleanly every time. A road test confirmed it started first time, cold and warm, with no clicking and no hesitation.
The outcome
A quick, clean crank, the engine catching first time every time, no clicking, and no intermittent no-starts. The C180 went home with the starting sorted. A worn starter is one of those things that's easy to live with until the morning it leaves you stuck, so changing it before that happened put the car back on a firm footing.