A head gasket job is not impossible but should be done with skilled supervision, even on an M30. It is a 2 day job unless you are really good, but w old m30s, it will only be the gasket in most cases, esp. if the engine was not cooked. People used to working on other makes can fail and there are plenty of those that end up at the boneyard. Also, they should use water until the engine is sealed up (after a few days driving and the re-torque to stop coolant going into the cylinders, getting vaporised and destroying the cats)