well - what years and what price range are you looking for?
85-88 1/2 : 3.5l V8, LT230 (with locking center diff)
89-95: 3.9l/4.2l(LWB), BW 1361 (viscous coupler in the center diff).
93-95 will have ABS and airbag suspension - more stuff to go wrong and more expensive to fix, but the handling is way better than on earlier ones. Also, IIRC, in 93 the 10-spline diffs/axleshafts were changed to 24-splines. 95 will have serpentine belt arrangement similar to the Disco - it has some +s and -s, but overall it is simpler than V-belt setup. Replacing the p/s pump on an earlier Classic is a nightmare - but it's a walk in the park in the 95+.
As far as overall condition goes, look for telltales of poor maintenance - multiple oil leaks, coolant leaks, power steering leaks, oily/greasy swivel balls, etc. Lift carpet in a few places (need to unscrew the aluminum door sill panels) - you may see rust underneath. Mine was full of sand - the PO was a surfer dude, cutting every possible corner in maintenance and repair (which I didn't recognize until later). If you come across the gas receipts, see what grade gasoline the PO was using - regular may indicate a cheapskate.
Take a vacuum gauge and measure manifold vacuum while in gear, at 1800-2000 rpm (crank the parking brake to the max, and step on brake pedal very firmly). If the vacuum goes down to 2-5 inHg, it indicates melted/plugged cats and muffler = $$ and work.
Drive it around in traffic until temperature stabilizes - and see if it loses coolant. If it does, you are looking into one or more of the three: blown head gasket, cracked head/cylinder liner, or slipper liner. Head gasket isn't that difficult or time-consuming to replace and is a great bargaining point, but you'll never know which one of the three possibilities happened.
In a properly running RRC, all malfunction lights (e.g., EFI, Check Engine, Low Coolant, Transmission temp, etc.) should illuminate upon the start-up, and go off when the engine is started. If any of them doesn't come on, suspect it bypassed.
anyhow, rangerovers.net will have nearly all the info you need.