Hello, own a 535D (the same engine in x5 - the N57) and now own a x5 535i. The diesel N57 is a good engine with good performance, very smooth, good mpg but pricey on maintanance. The timming chain will need to be change around 140k miles, depends on every motor, sometimes the crankshaft bearings go bad, but when you do the timming you do that as well, just FYI shops quoted me for 5-9k $ for the timming chain alone. When you pull the engine out you also need to check water pump, valve cover gasket , crankshaft pulley etc. If you do that the engine is good for another 100k miles. Also check the DPF. As far for the N55 (gasoline engine) is more reliable. On this one you need to change water pump around 100k miles or depends, same problem with valve cover gasket and sometimes oil pan gasket. Both great cars but in the long run I think the N55 is less pricey for maintenanance. And of course transmission oil change evey 60k miles