I just did my front and rear diff oil, brake flush, CDV valve.
Oil from bmw was few bucks more but i know it works.
i got transfer case oil but couldn't get to the filler cap (horrible design).
my tranny oil was changed so didn't touch that.
car has 112.000 km.
i would also do the sparkplugs
as for the tranny fluid get royal purple it is better than oem.
so you have few things to do and YES parts are so much cheaper in the US, why not save few bucks. we get shafted here in canada on the same parts basically.
from there on, mod time but keep your fluids up to date and don't believe the lifetime oil crap. you should see the oil that comes out of the cars.
as for the engine oil 15.000 km is absolutely not acceptable. i wouldn't even go more than 5.000km
oil is essential for the engine and even though the bmw and the latest oil molecular structure can withstand a lot of abuse 70-90 bucks every 5k is nothing to invest.
i change my regular oil on 3.000km on my other cars and never had any issues.
the bmw gets it at 5.000km