Hmm, not sure about that. When I bought mine, it looked great to me but it still needed a full exhaust system and a head gasket. I can't remember what it cost me to fix but it wasn't far off the purchase price of the car.
Without wanting to sound funny, head gaskets don't normally go overnight, and if it needed a complete replacement exhaust it would have been pretty obvious from a visual inspection
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Having said that - the original MB exhausts are very long lived - might need a backbox every 5-10 years, but the downpipes and mid sections last for a very long time - 15-20 years I would guess.
As with any cars, you need to check them over - well worth taking someone knowledgable along if you're unsure. A head gasket is something I've already mentioned - parts are cheap (less than £30), labour would either mean a weekend of DIY or paying for someone's time/expertise otherwise
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Will