Known issues with high-mileage BMW and Mercedes?

Our experience is that older BMWs suffer problems with things like the plastic fittings on the engine, the plastic radiator tanks etc. that can add up to a sizeable sum of money when you have to replace them. One car, an older 3-Series that had done around 250,000km developed a problem with the power steering that had mechanics completely stumped. After four visits to the workshop, having replaced the steering rack, the power steering pump and many other steering parts, it was finally found that the fault was in the steering column. That was replaced and the problem fixed, but it cost more than $2000, and that's using second hand parts. That said the engines and gearboxes, etc., are bulletproof. We don't have the same feedback on the C-Class, but remember all cars have problems; no car is completely free of trouble.