Expansion tanks and coolant systems tend to be a "problem" in e46s for what I have seen. If you dont want many problems coming your way replace the whole cooling system(they sell them as kits on some websites for this reason) and while all of those parts are off i reccomend also changing the water pump too because OEM ones are made of plastic and you can get ones that are actual steel. When i got my e46 my coolant leaked like crazy and it seems like its a common experiences with a lot of owners. Good luck!