I'd have to disagree on the reliability of the 6G72. I had the FWD for Chrysler version though. I gave the minivan away with over 230,000 miles and it was still running strong. And the DOHC 4 valve twin turbo version offered in the Dodge Stealth and Mitsubishi 3000GT were stout motors with big HP numbers. I will agree that parts availability in this country for that motor are fairly sparse. Australia offers fine performance parts for all versions of that motor.A very common problem with the 6G72 is lifter ticking. But is pretty easy to replace. Also, the late 80s models had exhaust valve guides that slipped and let oil into the exhaust chambers. Not as easy to fix, but cheaper than a ring job that MANY shops pushed on people at the time.
SoCal is a real HOTBED of offroading activity. Rock crawling, desert running, expeditioning... I'd look into some of the expeditioning sites for upgrade info for your Montero. When I lived in San Diego I could hardly swing a dead cat and NOT hit an offroader of some sort. Of course, when I moved to the High Desert, they were EVERYWHERE.