The only Range Rover worth considering for any outdoors activity is the Classic. They have a better ride than Toyotas, and with tender care they can actually last quite long. You do have to spend time maintaining them, yes, but parts are easy to get at reasonable prices.
The Vauxhall Monterey is basically an Isuzu Trooper. Well, let the unthinkable be told... I personally prefer going offroad in my Trooper than in any of my LC. More comfortable, nimbler, generally speaking a better offroad drive. They are built tough, on par with a Toyota. The only drawbacks: there is less room in the boot, and they look less... Big and tough.
The only non-stock parts installed on mine: a set of rear Range Rover Classic springs (lifts you 50mm, with same spring rate), manual hub locks, and nylon spacers for the body.
Now, "offroad" in my neck of the woods means weeks without seeing a road, and from sunrise to night across the bush in elephant country. City-toy 4x4s need not apply...
So, Range Rover? Unless you're an enthusiast, stay away. Monterey? Give it a good look, that's a lot of very capable car for usually little money.