For the relevant passage re Denethor:
Gandalf said:
[Denethor] is not as other men of his time, Pippin, and by whatever be his descent from father to son, by chance the blood of Westernesse [Numenor] runs nearly true in him; as it does in his other son, Faramir...
Now, as to Dunadan/Dunedain: I have always felt that Tolkien is difficult to model in D&D because his characters derive their "powers" so much more evidently from race, lineage, divine grace, or flat-out luck than from straightforward experience. While I share some other posters' discomfort with the classist/racist elements that emerge (and remember that LotR is in part a mammoth response to what Tolkien felt were the undesirable arriviste notions of industry and modernity), I also think that it WORKS in LotR, and that D&D, as well as much [excellent] fantasy literature, does, for better or worse, take a lot of this from Tolkien (hence the five different human-like PC races, each embodying particular tendencies that could easily be seen as subsets of the human mindset, and "inherently evil" races like orcs and goblins). I'm OK with it largely because my players and I know the pedigree of this and have fun playing with it and using it to our narrative advantage.
But this is sorta general. A truly Tolkienesque depiction of High Men really starts with the Elves, who either need to have the half-celestial template (minus wings) slapped on (a not-so-bad facsimile of elven abilities in LotR) or some serious stats that would probably warrant a +2-3 LA. Give the Half-Elves either the same stats as the Elves, or allow them to take the path of Men and run roughly an LA +1 template of some sort (stat boosts, low-light vision, some resistance to disease and increased longevity). Use that template for the Numenoreans, and then either continue it for the serious Dunedain (like Aragorn) or use a feat that confers a watered-down version of the benefits for lesser Dunedain. Those are my thoughts.
One thing, of course, is that all of these folks who actually DO anything (Elves, Men, and Half-Elves alike) should have LOTS of character levels to reflect their tremendous age and power.