So for the M-Sport package question: In the "comfort mode" (which is the default) the ride is very comfy, turning on sport mode tightens everything up for more spirited driving. The only thing I would recommend about this is to go to a dealership and sit in an X3 with sport seats. Some people (like me) love them, others not so much. Its an individual taste.
I *highly* recommend you get the ZDH (dynamic handling) package along with M-sport for the steering (you will get performance control and dynamic dampers with M-sport). It is my opinion that sport steering mode is the best from a road feedback point of view.
I'm actually about to take a 3,000 mile roadtrip journey to the midwest so I'll have good feedback by the end of the month on how it is for long rides.
As for driver assist plus: BMW didn't put blind-spot alerts on any of the X3. It is not available. At all. Lane departure warn is there. There is a button to turn it off if you desire (it is also not active by default under 40MPH.
Make sure that on the Driver Assist Package that you select option 3AH (Rear Cam with Top view). It is indispensable in parking situations. 3AG (rear cam only) is the default.
If you are not getting it I super duper recommend the technology package so you have head-up display. I thought it would just be a cool gadget to get when I ordered mine, however it has transformed my driving experience, and don't want a car without HUD ever again.
Finally, this has bitten a few people: Verify on the configuration that the exterior trim and roof rails match and are what you want. I dont think it is an issue for the M-Sport package, but if you get it or not, make sure they are right. I think that is 760 (black shadowline trim) and 3MC (black roof rails). Or, 3MB for aluminum trim and 3AT for the aluminum rails.
Hope that helps!