My car is a 'facelift' 2006 saloon built in MB's South Africa plant, and it has no rust.
In fact it proved very reliable, with only a couple of minor faults so far.
What engine were you considering - petrol or diesel? 4, 5 or 6-cylinder (I am assuming you are not looking for the formidable C55 AMG...)?
Petrols are cheaper and there are plenty of them, lots of C180 Kompressor and C200 Kompressor around with low mileage. Low mileage cars are recommended because the M271 engine (4-Cylinder petrol) is prone to timing chain issues at 80-100k miles.
Diesels are more sought after, especialy the 220 CDI which offers good economy with reasonable performance. Most will be high milers, but in the case of the diesel engines it is not much of an issue - though service history is paramount.
Also note that there are different trim levels, some models are 'sporty' with lowered and stiffer suspension, wider low-profile tyres, and brushed aluminum or carbon effect interior, while others are geared more towards comfort and softer ride and have wood-covered central console.