Fuel tank sending units are probably shot. Common on many MB models. If the problem only began after the repair, then something might not have been connested properly.
The M111 engined W202 from 1996- on a very reliable car. No wiring harness issues, no headgasket problems. Double row timing chain that is long lived. Reliable AC system.
The 16V four cylinder is tough and pretty much problem free. I know a few C220 owners with high mileage, and report few driveline issues.
The problems you might have include:
1. MAS (Mass Airflow Sensor) is an issue on many late model (OBDII) makes, including Mercedes. Thankfully, the MB MAS is a fraction of the cost of most others. (Our Mazda 626's unit is $1300, most MB's are about $300-500)
2. Fuel gauge sending units. Heard a rumour oxygenated fuels cause premature wear.
3. Evaporator temperature sensor. Can cause the AC to on the blink, but is a relatively inexpensive thing to fix.
4. Wiper might begin to hang up. Don't let the dealer sell you a $1200 new unit. It can be lubricated.
5. Fuel pump relay - this you already know.
6. Lower front ball joints wear out prior to 160,000 kilometers, often earlier. Worse if your roads are bad.
Change the oil and filter often and you'll keep the timing chain from premature wear and make all the other lubricated bits happy too. Change the tranny fluid and filter every 50,000 kilometers.
Ignore the fellow at the parts counter. But, it's not a Corolla either. These cars need care and attention. If you want a Maytag, shop elsewhere...
1998 C230 "Black Betty"