Usually Tires required to be changed after every 3years (by RTA/UAE regulation) and 40000kms (as per manufacturer’s warranty). Even if your spare tires was never been used there's fair chance that tire quality, drastically deteriorated. So if any given day you need to the help of spare tire, there's a possibility that it will cover only 10km and will tear apart aswell. My personal advice would be never to take chance on tires & brakes consider the local high temp climate.
2- If you are wishing to change size and brand of the tires especially size then make sure you ask Ford local service advisor to change/ adjust the gear ratios accordingly. In this particular case say you have changed from OEM meager 245/65-17 to 26 to more appropriate 265/65-17, even then you should buy a full set of 5 tires as the combination of different aspect ratio will not allow the gears to change normally. Frankly speaking it’s a very unpleasant situation.
3- On my ’07 Ford Explorer EB, I have switched to Yokohamas Geolandars HT/S and increased the tires ratio from OEM 245/65-17 to 265/65-17. With personal driving style as aggressive, this change is poles apart. It literally transformed the ride of the car. There’s so much confident driving now than before when car was shod with BF Goodrich’s lifeless unsupportive tread design. A flaw or a compromise done by Ford maybe to gain extra/ optimum fuel mileage, but to me there was no difference, except that now the steering is bit heavier than before.