I have a QX56. If it's well maintained, should be no problem. The obvious difference with an Armada is that the front and rear bumpers are completely different. So don't wreck your bumpers as they are very costly.
Other parts are same as Armada, so you won't have any problems there.
Also, as far as I am concerned, both Armada and QX56 are not true breed 4X4 like the Patrol, but it holds itself well on the terrains of Qatar, both on and off road.
Ever considered the new QX56? It's an upscale version of the new Patrol.