as the car is out of warrenty then officially Mercedes aren't obliged to offer anything at all
however if you have a full MB service history and can show the car has always been maintained with the correct MB parts and fluids then they will have a remit from head office to make a contribution towards the cost
what might be a better compromise is if you were to source a good used engine and they fitted it for free
if they are prepared to offer £3000 then this should cover the cost of a used engine and the labour required to remove the old and fit the replacement
however it is unlikely that they will offer any gaurentee on the engine by going this route
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