I know there are a few 1st and 2nd gen Explorer builds here and there, at this point the issue is going to be the age of the platform and how it's been used.
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Older Explorers sell very cheap. This is both a blessing and a curse: Cheap vehicle means you can get into it for less $$ and have more to fix it up.
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The down side of a cheap vehicle, though, is that cheap vehicles are often owned by people who treat them like cheap vehicles, neglect maintenance issues, and even let minor problems become more serious through lack of attention.
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I think a 3rd gen Explorer could be a decent vehicle for mild off road travel, but the question would be, by the time you get one, fix all the little issues and then get it to where it has some off road capability, how much money have you spent, and what ELSE could you buy with that money?
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One of the reasons I think that mid-sized body-on-frame SUVs have all but vanished from the marketplace (last ones standing are the 4runner and the JK Wrangler, at least in the US market) is because the unibody crossover can do just about everything that the typical suburban family would do with their SUV anyway, and for those who need a genuine BOF, truck based SUV, the full sized models like the Expedition, Tahoe, Armada and Sequoia, do just about everything that the mid-sized vehicles could do and really only have a very small MPG "penalty."
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As P Nut said above, 20mpg on the highway is absolutely do-able in a Tahoe, and I can verify from experience that my 2004 Suburban has gotten 19mpg on a long highway trip at least twice. The place where a full size vehicle will kill you on MPG is if you are driving in city, stop-and-go or suburban driving. (My 'Burb has a hard time hitting 12mpg when used as a daily driver, and that's pretty easy to understand - every time I come to a stop, I then have to get nearly 3 tons of steel moving again!)