C350 W204

I'm considering buying a 2013 Mercedes-Benz C350 GCC Avantgarde (M276) with 250,000 km.

I test drove it and noticed a slight engine misfire/ hesitation. The seller said the car has been sitting in their yard for about 6 months without being driven, and the issue wasn't there before it was parked. They also said the problem is a "timing issue inside the valve cover" and they'll fix it before selling it.
The check engine light is not on.
Is this likely to be a minor issue (camshaft adjuster/ timing actuator, etc.) or could it indicate a major engine problem? If they repair it properly and it drives perfectly afterward with no fault codes, would you consider it safe to buy?