Overheating can cause the alloy head to warp. This may be caused by a bad viscous coupling leading to overheating in traffic. Non-UK cars with air conditioning use a two speed fan in front of the radiator, this can be a further cause of overheating in traffic. If the engine seems tilted forwards or out of line with the air flow meter housing suspect failing frount engine mounts, not expensive. Alternator rubber mounts fail around 130K, costs pennies and takes an hour to fix.
Circlip pliers are handy.
Despite all this the M30 is a very reliable engine. It's chain driven and unless it becomes noisy the chain will last the life of the engine. Many people advise a new timing chain at 100K miles but MW have no such policy. This is an expensive job and unless the car has a lot of track time many owners leave it unless it gets noisy. All are very reliable.