There was an optional size of 17" tires factory on the X2 so that should be fine. Again as rounderman mentioned if you get the m35i it's a different game completely and the 17" tires I'm almost certain will not fit.
You'd be best to contact bavsound about compatibility, but again, I have a feeling it would be okay to switch over. You may have to code something in however depending on the X2 having the hifi sound or HK system. I'm only saying this because I have noticed that everytime I look at the amp upgrades they pretty much work for every car.
BM3 you are mostly out of luck. Also if the dealer has updated the software it will lock the DME and no BM3 flashing would be available either. I also believe that the X1 and X2 also got the short end of the stick, and it's not listed under any BM3 tune anyways.
I agree with rounderman in terms of you might not increase the size too much, but that may be a good thing. I'm assuming your f33 is a 2 door and not a GC car with 4 doors. The X1 would have more space, but young kids are still small so if you like the X2 look more I certainly wouldn't be afraid. If you need to put in a double stroller, I don't think it would fit either X1 or X2
