From the factory BMW made them out of plastic to save weight. They work fine until they fail. Realistically the factory coolant system is sub par to the needs of the car. Replacing the rad and expansion tank, water pump and thermostat housing with aluminum ones is the best bet. If the car already has aluminum ones that means they have been replaced in the past. Which is fine, but jut make sure the engine wasn't overheated and blew the head gasket. If their is coolant in the oil, which will look light brown. Almost like what coffee does when you add creamer. That usually means their is a head gasket problem. The part is about $120 but the labor is what's expensive.
I have the M52 engine in my 328i and it's still the stock radiator and expansion tank and I have 262,000 km on mine. The water pump and thermostat have been replaced though. Preventative maintenance is the key. If you see cracks or anything that doesn't look right then replace it before theirs a problem and you will be fine.