To be serious first consider that it's a pretty old vehicle and that it will no doubt have had a hard life and been owned by some people who didn't look after it. So you really need to check everywhere for failing components and rot in anything metal. If you're new to series landies a test drive will make it seem absolutely awful. Slow, underpowered, dodgy steering, little or no suspension movement, and you'll be deafened by the noise. Engine wise just do the usual checks that you'd do on any vehicle, ie smoke, fluids, does it start ok when cold, any nasty noises etc. Remember these engines have been around a long time now but many have been looked after and/.or rebuilt and they are basically good and reliable. Don't listen to the old " yeah but they all do that" line from the seller, they don't all do it only knackered ones. If at all possible take someone with you who's familiar with old landies and knows what to look for.