You'll enjoy the driving dynamics of the X3 for sure; I know I did for the 2.5 yrs we had our diesel X3. They are surprisingly easy to maintain, parts aren't terribly priced, lots of great options for fluids and a very helpful community.
As for driveline fluids, I wouldn't bother changing until 60K and then stick with 60K (100K km) intervals. I changed both diffs and the transfer case fluids at 62K on ours and they still looked/smelled good. That ZF transmission is a solid unit that'll serve you well.
However if yours is an X-drive, you'll want to keep an eye on the rubber flex disc just behind the transfer case (I think they call it a "guibo" see
here) in case you start feeling some vibrations or rumbling down low.
Also, not sure if you're running the hideous "run flat" tires, but if so I'd suggest swapping to some good UHP all-seasons when you're due for tires. You'll be amazed at how much better it rides, handles, and generally how much quieter and smoother it feels.
Lastly, don't know if anyone has suggested this but if you don't have a BMW specific scan tool yet, get yourself the 2 best ones:
BimmerLink and
BimmerCode. They both work with Veepeak wireless OBDII dongles, among others. Probably two of the best purchases I made for my X3d that paid for themselves many times over.