As far as motors go, people that know more about the different ones can give you better advice on what to look for etc. Look at everything, bring a magnet in a cloth to look for bondo, tapping body panels also gives you a good indication of there condition, LC's boxed in frame rails seem to hold alot of crud, look at the frame closly and check for major rust and holes, the drivetrain is pretty bullet proof, check for excess wheel play buy rocking them back and forth, some is normal. Look at all yer fluids and as the person about serivice intervals on the truck etc. These are the toughest all wheel drive trucks in the world, but there are select things that need replacing after time and abuse. Drive it with the windows up and really listen on ashfault at around 100kmoh usally gives you a good indication of how solid a rig is.
From the sounds of reading the fourms for quite some time, all motors arew tanks but have a few ups and downs, some of the new 6 cylinder diesels need main bearings after about 200 000 or so it is more a precautuion then a nessary thing, and by the sounds of it isn't very hard to do and are quite powerful fast motors, the 4 cylinder diesels are gutless wonders but built well, unless you overheat them which causes head problems.
all in all it is just time to look at every little detail, don't be rushed by the seller take your time and look at everything, check for excess play in drivetrain,