Sort off. To see if the thermostat housing is aluminum you need to remove the cover over the alternator. On the M52 the expansion tank is to the left of the radiator if yor facing the car. It's green so it's pretty easy to spot. On the M54 I think it's on the right hand side and it's black. The thermostat housing is above the water pump.
The pictures in this link are from an M54.