If the expansion tank cracked I'd be worried about the thermostat housing. It's closer to the heat of the engine / exhaust and it appears to be the same plastic. The water pump is a no-brainer when you're there, as most of the work is removing the t-stat housing and draining / filling the system. The radiator has to come out on an E90 to get to this stuff.
Really not liking the engineering of the newer BMWs