Hi Draos .
Going from what your mechanic says looks like a headgasket problem .
This is what can happen if you overheat the engine .
This would explain your symptoms.
What he means is the head will have to be checked for cracks and to check if the head is warped ,if warped will have to be machined .
Have him do a wet and dry compression test to make sure the rings are still good, if the bottom half proves good just replace the head gasket and have the valves reseated.
This should be done if headgasket being replaced .
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Kind regards Greg