They can weld the surface of alloy heads fine, but a lot of cracks are too hard to fix without risk of a split at a later date. I'd insist on seeing it when it comes back from the machine shop. It's good news the block is ok, but how can they be sure? What broke when it made that horrible noise??? Cylinder 6 is the one that goes wrong- as the rear one is the hardest to get to when the engine is in the car. If the headjob was not done well last time this could might be a where problems occur if there was any foreign material in the gasket. The gasket should be carefuly inspected on removal, and the (renewed) surfaces, when brought back together, should be perfect to the touch and eye.
Check the parts listing on the valvegear section in www.realoem.com so you can go over the head and make sure it is all there and the right way round (are they good w BMW heads?) particularly the little bolts at the rear are installed the right way round and with the right o-ring. These are damn near impossible to check once the engine is in. Important are the coolant orifices so the coolant can circulate into the head okay- with bad coolant and problems these tend to block with scale and oxidised iron from the block. If your shop or mechanic isn't very familiar with M30s you should do all this unless they are really methodical people and highly competent that will check the right book on all the little things and check their suppliers work over well; a lot of mechanics expect the head to come back ready to go on and will only notice something that leaks or is missing, not a nut screwed in incorrectly... or an oilsprayer bar installed around the wrong way. The big problem in Aus (you may know), apart from saying after the fact that 'it's not my fault but theirs', is that any of us used to working on local cars have trouble changing our approach, the BMW way is very specific and can seem silly and weird to people that do not 'know better', if u comprenendt vhat vee mean, ya?
Also, make sure the mechanic is guaranteeing their work so you don't have to npay if it needs to be done again. Best thing is to be paying a fixed fee (from someone who knows how to compelte this exact job of course)...