I had a 2006 W203 for 9 years. But not a V6 petrol, so I can't comment on the engine.
Only 'common' issue that I can think of was the pilot bush (electrical connector) on the 5g automatic transmission, which tends to develop a leak which requires replacing it, along with the ATF in the sump.
The diff at the rear can develop 'sweating' from the drive shafts, it does not require urgent attention (the amount of oil lost is minimal and the diff is not very sensitive to the oil level anyway), but it will need to be fixed at some point, and replacing both drive shafts seals is quite labour-intensive so will be costly (unless done DIY).
For some inexplicable reason, most if not all MB engines seem to suffer from premature thermostat failure, so I would check that (the engine should reach 80 degrees within the first 10 minutes of a test drive, and stay above 80 for the duration). The cost of replacing the thermostat can vary greatly from one engine type to another (as an example, it was around £100 for my M271, but around £800 for my M274... dealer prices).
Rear dampers tend to do, but it's normal as they are consumables. Springs can break. So the suspension needs checking.
Post-2003 cars are galvanized, so they rust less, but this is not to say that they don't rust at all.
As greenmanslk pointed-out, the basic Audio20 is very... well, basic, and the optional COMAND NTG2 satnav is outdated. Also, all facelift W203 have the phone button on the steering wheel, but it's meaningless and you won't have Bluetooth telephony unless the Convenience Telephony option is installed. It sits under the centre armrest - if you lift the armrest, you will see a shallow storage tray - if you can lift the shallow storage tray and there's a deep empty cavity under it, then the car does not have the Convenience Telephony option installed and won't have hands-free telephone support. If, on the other hand, you can't lift the shallow storage tray, then the car will have the Convenience Telephony option. Either way, the outdated Audio20 or COMAND units can be replaced with a modern Android-based system that will also have Bluetooth telephony, if that's of interest.