A noisey tappet is not gonna render the vehicle unusable - maybe a tad under powered, noisey and annoying....but swapping out an engine for the sake of checking the tappets is a little extreme!!!
Also, if you need reliable (even without noisey tappets) a Range Rover is not the car for you!
I am Range Rovers biggest fan, I have owned many and have been driving them for over 20 years, but even I have a spare car for when the time comes it throws a hissy and refuses to play the game. They are cars for enthusiasts not as a daily driver - cos, yes they can be driven daily and there are many very good ones out there - but similarly, there are some shockers too.....and it is hit and miss so plan for it!