Rust is the usual landy killer - check the chassis thoroughly from underneath and make sure the bulkhead is solid. Chassis and bulkhead sections are available as replacements if you're handy with a welder. Then it's just wear and tear. The engine shouldn't smoke unduly when hot and should start easily even in this cold weather. The gearbox should change into all five gears without too much clonking and run without too much noise - these boxes do produce noise and the gearchange isn't as smooth as a modern vehicle.
Make sure the hi/low and 4wd is working (see the "what do all the levers do" thread).
Look for play in the propshafts and diffs and the steering - an inch of play at the steering wheel circumference isn't unusual.
Check the electrics - older landy's have often been bodged horribly.
Look for pitted swivels that leak badly.
Look for leaks generally - landy's seem to weep oil from everywhere - and familiarise yourself with smell of ep90, 20/50/, diesel and brake fluid so you can tell what's leaking.
Just about everything is fixable and most parts are cheap and available. You will need a parts list and manual (downloadable on this site somewhere) and a good set of tools.
Oh, and a note from your psychiatrist certifying that you are mad enough to buy it......