B34RD

Hi all, I am new... I will be purchasing a 2008 X5 3.0d (SE or M Sport not sure which yet) hopefully a 7 seater for the right price. My question is; what do I need to know, what do I need to be checking, what do I need to be walking away from, what are the common faults, is it a decent car etc... you get what I am saying. I have no real car knowledge, but will be looking at an X5 with an auto electrician. Any help is much appreciated. Regards Chris.