The over heating issue you seem to be experiencing can be caused by factors such as a fan clutch that fails to engage or a radiator that is no longer able to carry away heat due to deposits that form due to age and possible lack of preventive maintenance (scheduled coolant flushes) As far as the thermostat going missing, there is a school of thought that says "remove the thermostat and there won't be any more problems with over heating". Nothing could be further from the truth. The t-stat is at the heart of keeping the temperature constant as the flow can and will become "confused". Especially in a BMW. I hope this is helpful, repost if more assistance is required.