BMW Repair: 1997 BMW 528i overheated, water pump impeller, fan clutch

Question
6 months ago I had my thermostat replaced due to overheating, at the
beginning of last week when I would come to a stop the car would running
hot
but only when at a complete stop. A few days later it was in the red while
driving there was a knocking noise and then nothing. After being towed to my
mechanics i was informed that the radiator split, thermostat is done all due to
the overheating and once that is replaced then the mechanic can inform me
what was wrong to begin with. His current thoughts are either the fan clutch
(300 dollars?!) or the head gasket (3000 dollars??!!). I know very little about
cars and don't want to be taken for a ride, it is just hard for me to
comprehend "everything" destroyed due to an overheat.  Thanks for any help
or advice you can give.

Answer
Amie, BMW's are VERY sensitive to overheating. If the headgasket is not blown from the overheat I would look to the auxiliary electric fan in front of the radiator for correct operation. Fan clutch is a possibility, but less common. Check water pump impeller and make sure it is not the plastic type. If the water pump impeller is plastic then have it replaced with a composite metal one. Aluminum can warp and crack very easily when overheated. Tell your mechanic to look over these items.
-Evan