BMW expects the xDrive30d to be the X5's big seller, and with an on the road price of just under £50k it's handily cheaper than an entry-level Range Rover Sport. It's also much better than the mediocre base Porsche Cayenne diesel.
Running costs should be acceptable on a comparitive basis and reliability should also be good. The build quality is of a high standard and the engines are well-proven units.
The most economical option is the 25d, which is claimed to average 50.4mpg, but if you want four-wheel drive then that drops to 48.7mpg. The less strained nature of the larger diesels may make it easier to average higher figures in the real world than the smaller engine option, however.
When it comes to specifying your X5, stick with black leather and aluminium trim; wood veneer is likely to cause a problem at resale time, especially if you've got one of the more sporting models. Have the Harman Kardon audio, enhanced Bluetooth and adaptive LED headlights, too. There's no great need to opt for the adaptive dynamic suspension either, unless you're intent on seeking out the most capable X5 possible.
Most models should also retain their value in a similar fashion to that of the Range Rover Sport, so in other words very well, which is an impressive performance.