Hi Mark
Welcome to the Forum
Wow that is a serious step up in car, your mechanic friend is right to a degree, you are contemplating a complicated car that carries all the additional costs of parts and servicing that go with it against a car that has a much lower operating cost.
That said I have two friends with X5 5.0d both like me enjoy doing their own servicing and repairs, that makes running these cars very reasonable in my (and their opinions)
Make sure the car is smooth in operation plenty of power is an understatement 8 speed box should be effortless and silky smooth in changes, make sure everything works as it should test everything (twice) Tyres make sure they are correct * rated tyres and have plenty of life replacing them is expensive and essential not to cut corners with tyres.
Get yourself a BMW capable Diagnostic Fault code reader (I have INPA/ISTA and BMW 1.4.0 note 1.4.0 is pre 2005 models ) If you are competent with spanners you will be fine
Dave