I've written a word or two re buying E53's and based on me ending up with a 2002 4.4i Sport
Click on this link
http://www.x5drivers.co.uk/viewtopic.php?p=11192#p11192
The starter list by FSETH is good advice as is listening for and feeling for bush/suspension rubber wear, checking tyres and brake discs/pads plus dash display and all electrics including all toys.
The wrong car can cost thousands to sort so beware of the bargain car in need of tlc unless you are able to fairly accurately assess costs involved to get it to standard aceptable to yourself.
Good Luck and let us know how you get on.