If you're a member of a motoring organisation they usually offer a service of sending someone out to check over a vehicle you're thinking of buying. Another thing is that they publish the checklist of things they look at on their websites. They're very useful as reminders of what to look for before handing your money over. For example I looked very carefully for chassis and bulkhead rust, and looseness in bearings and transmission components when I was buying mine (found a loose wheelbearing that the garage agreed to replace for me before I picked it up) but I completely forgot about basic things like brake lights and windscreen washers. Most things on a Land Rover can be replaced or repaired, so it's about imagining how much work will be involved and whether it seems worth it in relation to the purchase price. It depends on what tools you've got to hand and what you're good at doing. If you haven't got a welder it's a bit of a ****er if you buy one with extensive rust for example. And for vehicles mostly made out of aluminium, they certainly can rust a lot. If you've got spanners and hammers and punches you can probably replace most bearings and seals, though for heavy items like the engine and gearbox, an engine crane is useful.
Of course, you can check the MOT history online which can give you some idea of the faults that have been detected and whether they have been repaired.