And some more "out loud thinking"...
Engine breather/oil separator. Not sure if you have mentioned this elsewhere but if you haven't replaced this yet - it's way past time. The original design oil separator is a fibre/paper filter element that becomes very clogged and blocks the engine breather. Give enough time (and oil) it could collapse and allow un-trapped oil etc. to pass into the inlet tract.
The fibre/paper element was superseded by a vortex-type element but fitting this requires replacement of the whole engine breather assembly - or so I'm told. I did this but on review I think I could have fitted the new vortex element to the old breather. The breather assembly does have some minor design changes so in the end I'm happy that I replaced the whole thing.
Here's the OLD style assembly...
And the new one...
I got mine from just down the road from you...
BMW E46 E60 E61 E65 X3 E83 X5 E53 M57 N ENGINE CRANKCASE FILTER OIL BREATHER | eBay
sn_autohaus are great folk to deal with.
It's not a bad job replacing it and it's a good idea to replace the air filter while you're in there.
Here's what mine looked like after 200,000km.
And here is the new one in place (shiny new black item at right rear of engine - left side of photo).
Another thing you might want to look into is a check to see if any oil is making it's way up from the turbo. Pop off the outlet pipe from the intercooler at the intake to the inlet manifold (where the EGR meets the inlet manifold - known as an EGR Valve) and clean it all down, removing any oil etc. Then run the car for a day or two then re-check the inlet (EGR Valve). If there is a fresh coating of oil all over everything in there, you may have the smoking gun. A little oil deposited by oil vapour is normal but it shouldn't be enough to make the engine smoke at startup like you describe.