No red flags at all, they are solid. I would go beyond BMWs service regimen and change the coolant every two years as well as change the transfer case fluid, ATF, and final drive oil at 60k miles, but that's it. They are even fun to drive; I've been tempted by more than one M40i.
The B48 I4 is a great engine and the ZF 8HP is about the best automatic available. Me, I would want the M Sport, Adaptive LEDs, Cold Weather Package, and H-K sound. Regardless, they are outstanding vehicles- the X3 is the sweet spot of BMW SUVs.