I was speaking with a mate of mine earlier this week, he was telling me about a recent 4WD trip he had done with a bunch of his sons mates, there was about half a dozen vehicles, mostly old D1's, a Nissan, and one guy with a Disco D4, albeit a bit used, in his words.
Long story short, they did a really hard trail in a state forest, I've done that trail in years gone by, it is full on, believe me. My friend, whos D1 is hard core, as were most of the other vehicles, (diff locks, rock sliders, high lift long travel suspensions etc), well he was really impressed with the D4, it went everywhere the older vehicles went, (with a few scrapes and scratches), and some places only a few of the other more intrepid drivers would attempt. Done by the D4 with consummate ease, so no real problems with ability there, just depends on the depth of your pockets and how much you need to keep it pristine.
Though I think you will not be looking to beat the Disco up in the rough, they are a very capable vehicle out of the box and the amounts of cash you chuck at it is mostly optional.