The E70 X5M has the first gen S63 motor that suffers from most of the issues that the N63 has... which are many. Most of these were fixed in the S63tu in newer M cars (with improvements coming all the way up to the tu4 now found in the current X5/6M & M5/8)
He is better off buying a diesel X5 if going for something that old IMHO or go with a newer M if that is really what he wants. If he wants a family car (and he is likely not planning to do any off roading in an X5M LOL) he should sell the M4 and buy an M5 or M6 GC.
Other than that 4x4, preventative maintenance includes transfer cases and other parts of the drivetrain, gearbox servicing (most people will not have done this and the car is old now) outside of what you would do on any other car.