The N55 engine used in the F26 X4 35i appears to be generally reliable. There are several problems that may arise with increasing miles, particularly if maintenance is lax. The ZF transmission typically is fairly reliable for the first 100K miles and reliability may be extended with periodic fluid changes.
Consider the information at the link below:
https://www.bmwn55tuning.com/n55-common-problems/
"Despite the many misconceptions that BMW’s as a whole are unreliable, the BMW N55 is actually a fairly reliable engine. The four most common problems with the N55 are, in no specific order, the valve cover and gasket, water pump, oil filter housing gasket, and VANOS solenoids. None of these are necessarily major issues, and most of the parts are not expensive. However, and I think this is where the misconceptions about BMW reliability come into play, the repair bills can begin to add up if you always service your car at the BMW dealership or independent repair shops."
"Also, do keep in mind, this is not an exhaustive list of things that can go wrong with the N55, and simply because something is on the list does NOT mean it will indefinitely be an issue. With that being said, we are examining the “average” N55 and what goes wrong with the “average” engine. There are plenty of N55’s with 100,000 miles that have not cost a dime in out of pocket repairs, while others with significantly fewer miles may cost thousands a year in repairs. It all comes down to how well you maintain your N55, and to some extent, the luck of the draw."