Exactly right, a 250,000 mile 200 or 300 TDi engine (generally speaking) is nicely run in at that kind of mileage, providing that it has been well maintained. As for knowing how well the engine / vehicle has been maintained it's a matter of applying common sense and being thorough in your investigation. When checking the engine, start by having a general look at the condition of the engine. Check the oil and coolant levels, are they low? Is there oil weeping from or covering most of the engine? Fire her up. It should start immediately without any glow plugs, and you should see a puff of black smoke leaving the exhaust. Repeat a process of hard revs followed by an idle until the engine begins to warm up. Once the engine is warming up, remove the oil filler cap and place your hand over the opening. Can you feel any oily mist being blown at your hand? Next, remove the oil dip stick and press your thumb firmly over the end of the tube for five seconds, whilst someone revs the engine hard. Quickly release your thumb, does the tube hiss or is there any pressure build up / oil launched out of the dipstick tube? What you are doing here is checking for 'blow-by', or in other words, compression gasses that are escaping the cylinder either through gaps created by a blown head gasket, worn piston rings, cracks in the piston crown, cylinder block or cylinder head or even worn valve seals. You'd first have to diagnose the problem and then repair it before it's really usable. Observe the exhaust now. There should be absolutely no smoke at any time during the operation of the engine (besides start up). If you see black smoke when revving the engine, this indicates that the engine is being over fuelled, or it may have had the injection pump tampered with for a little more power. If you see white smoke, this would indicate either incomplete combustion of the diesel (atomisation of the fuel) which means that it's loosing compression somewhere or not firing properly. White smoke could also indicate water in the cylinders which ay be caused by a blown head gasket or cracks in the cylinder block or head. Blue smoke tells you that the engine is burning oil which is not acceptable and will cause nothing but trouble in the future, and ultimately lead to engine failure.
The next thing to do is take the Land Rover for a test drive. If the owner wont let you, there's clearly something that he / she is hiding or that he doesn't want you to uncover. Walk away from that landy and never look back. When you take one for a test drive, ensure that the gears are smooth, with no crunching when changing up or down (it is common in LT77 gearboxes with worn syncros) and ensure that clutch bite is always at the same level and not too sudden. Also engage low ratio and work though the gears. Ideally you want the drive to be very tight with no sloppy gear changes and no juddering or jolting under load and when coming off the accelerator.
Oh and make sure tat you hit the chassis with a hammer to ensure that it's solid. It should make a nice ringing clonk if it's in good shape. If it makes a thud with no ringing sound, or if your hammer / screw driver goes straight through the chassis, it's shot in that particular area so bare that in mind before you buy.
-Pos
