I think you could probably get by with a much simpler cooling system pressure test. You are having an overheating problem, check the cooling system first.
Check for all of the symptoms listed in the thread, milky oil, oil in coolant, bubbles in the expansion tank while running. If there is coolant in the oil, chocolate milky looking oil, stop driving the car or risk major bearing damage.
If you smell coolant in the exhaust, the car is overheating, and consuming coolant, the headgasket is the most likely culprit.
Current ride... '13 GTI Wolfsburg
BMW's from my past...
'92 535i/5 '89 325ix/5
'98 750iL '86 325(e)/5
'95 740iL '84 318i/5