Take a look at the hoses, water pump pulley, etc. If the tank failed they might also be worn out and on the edge of taking a dive on you. Cooling issues on BMWs are such a common occurrence we've put together a hand full of refresh kits, found
HERE, to help make getting everything back in shape a lot easier. If you'd prefer to just grab an expansion tank and t-stat I have those available individually as well.
HERE is a link to the expansion tanks and
HERE is a link to the thermostats.
ECS Tuning