Just following up here. Took everything apart and all parts were intact. Leak was coming from the somewhat loose connection where the hose connection fitting (#3 in the diagram) connects to the bottom of the expansion tank. I replaced the o-ring inside the tank where that part snaps in and now that connection is way more secure. Pressure test was successful and I've been driving it around without any leaking. Happy I didn't need to purchase any parts this time around.

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'05 X5 3.0
'06 325i
'90 E30/S52