The later 4.0 and all 4.2 engines have much improved tensioners which are unlikely to fail prematurely. The engines last very well; I have encountered properly maintained examples still running sweetly at 400,000 miles.
Of course, any piece of machinery can break or wear, so don't omit to check the whole car thoroughly and look for evidence of careful past ownership and maintenance.
We have only had one instance of a secondary 4.2 tensioner issue reported here. The slipper plate came adrift from the plate above the piston. These 4.2 ali tensioners appear almost bulletproof.
You have little to worry about with the 4.2 engine. They do require regular oil changing, the very occasional waterpump, thermostat and cylinder head temperature sensor, the odd belt, pulley bearing and perhaps on very early cars, the later drive belt uprate belt tensioner modification kit on supercharged models. Other than that, the 4.2 can be regarded as an exceptionally reliable engine. Any of the above items are quick and simple to rectify.