Before buying an X5 for towing, pick up the current issue of "WhatCar" and turn to page 94.
A chap there has written to the "Helpdesk" feature of that magazine bemoaning the near £2k (yes thats right!) BMW have just quoted him to get a towbar & uprated fan fitted to his X5!
In a nutshell, it appears the X5 needs additional cooling capacity to make it a decent towcar consisting of Towbar & electrics for £1175 and an uprated fan for £700! - If this work is not carried out, the warranty will be invalid should you have furture overheating issues whilst towing!
Sytner leicester have eventually dropped the total price of the conversion to £1150, but, I'm sure you'll agree that's an awful lot of cash for a sodding towbar!
FWIW, Audi's Q7, Landrover's Discovery and Volvo's XC90 all have no requirement for such a cooling system upgrade.
Food for thought & shame on BMW!