With regard to the coolant leak: If your expansion tank is not the problem, consider the o-ring on temperature sensor located on the lower radiator hose. This link gives a good discussion, though they recommend replacing the sensor for $30. When I worked on mine, I just replaced the o-ring. See post #19 in this link for the BMW part # of the green o-ring (though some people have used #31 danco o-rings from Home Depot).

One more excellent DIY with pictures in this link.

Hope that helps.