I agree with TMV, 6 cylinder with manual transmission will have fewer endemic issues overall. Ideally the NV125 transfer case (pre-iDrive), no on-board DSP, SatNav or fancy sound systems. For the ultimate (non-North American markets), buy a 3.0 diesel (184 HP) with manual transmission. This also avoid the diesel intake swirl flap issues (CCV & DISA valve on the 3.0 liter gasoline engine is less costly and usually won't kill your engine).

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2002 Euro X5 (E53) 3.0 diesel (M57)
5 speed manual with UUC short shifter.
Pre-xDrive NV125 transfer case (chain & gears).
Factory Xenon headlights, Spyder LED tail lights
Updated vortex crankcase breather.
Motorsport dead pedal, factory fire extinguisher, factory first aid kit, factory F&R mud flaps, factory PDC

Bilstein B6 shocks F&R
Redline oil has replaced "lifetime" fluids (F&R diff, manual transmission, transfer case and P/S).
Dimple magnetic drain plugs in all compartments.