The 85 % "rule" is just a guide and the CC say an experienced caravanner may go up to 100%
Nissan say 2000kg but that is what it is technically capable of and what legally it can pull.
Its not a good idea to do it in winds like we've had today crossing the Thelwall Viaduct for example.
The good thing is that you can tow at 85--100% without having a lower tow weight imposed on you by the manufacturer
If you look in PC mag Top Towcars several have a warning triangle "CHECK MAX TOW"
The towbar is not difficult to fit on the X Trail so any towbar fitter will do
I would get the Nissan wiring kit though.
It may be more expensive (there's a socket behind the trim level with the rear side window) as it will avoid the quick fit fitters beloved scotchlocks.