Have you checked the waterpump to make sure it's still flowing? Check with AutoZone and see if they have a tool you can rent that will allow you to pressurize the cooling system. Start off from here and see where that takes you before you throw a bunch of money into parts without confirming they're the issue. You have to remember (if you was not aware already) that the majority of the cooling system is make of plastic parts which are only failure points. We service a bunch of E39 5-series and have yet to come across one with a bad headgasket.