You're right that the in-town mpgs for a Hundy aren't great, but what mid-2000s big, cushy 3-row SUV does significantly better? None that I know of. And you can get 17-19 on the highway, versus the EPA Highway rating of 16. That, in combination with a 26 gallon tank, gives you decent highway range. More to the point, neither of your other possibilities is going to do a lot better. In '05 the combined MPGs were 13 for the LX and 15 for the Montero and GX, so it'd cost about $500 more a year in fuel for the Hundred, which in not insignificant, but the difference in purchase price will be a bigger factor.
I've driven many a GX and, no surprise given all the shared components, they're a lot like the LX/Landcruiser. In the 2005 period you're talking about, the LX/LC were actually a ways behind the curve development-wise, and the GX, with Downhill and Hill Start controls, KDSS (if you can find it; it was not a common option) and a bit more sophisticated electronics, seemed slightly more advanced than the LX. But those small differences are probably less a decision point than the styling and, especially, the rear opening. To my eyes, the 100 is the better looking truck--mostly because the GX has a taller profile, though that admittedly has advantages--and its combination tailgate/liftgate arrangement is my very favorite setup for overlanding. The single swingout rear door on the GX never appealed to me, in part because my garage stalls (indeed, many garage stalls) are too short to let you open the GX rear with the overhead door closed. Also, things like rear hatch tents, the Kelty Carport, etc. don't work with the swinging door. But many people obviously side with Montana Cruiser, though, as Lexus has stuck with that arrangement for over a decade now.
The second row seats in a Hundy are very well liked by occupants, important when putting clients there. The third row seats aren't great in the Hundreds, but the GX third row is almost unusable by adults because it is very cramped.
Aftermarket support for the Hundreds is better. The GX is more of a challenge, but there are very nice GX overlanders, and I know there's at least one with an extensive build thread here on the Portal.
Unfortunately, I can't talk knowledgeably about a Montero. All I could contribute is to point out the obvious facts that aftermarket support is limited and their probably aren't 10% as many dealerships as Lexus+Toyota.
Good luck in your decision making.
