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As tom says the main thing to look out for is rust, look everywhere, under carpets as well if they are still fitted. The hardest area to fix is behind the seats where the floor raises around the body mounts both inside and underneath, if this area is rotton on the underneath then you either need to be able to weld or expect a big repair bill. You need to lift the body off the chassis to repair this area, not that hard but quite time consuming.
Also I'd take a jack and jack up each wheel to check for movement in any wheel or kingpin bearings and to see if the bearings sound rough when the wheel is spun. All bargaining points £30 per wheel bearing to replace and about £50 for a set of kingpin bearings + labour if you can't do it yourself
Check for smoke, knocks or rattles on start up and make sure the engine has not been warmed up before you got there (a sign the owner might be trying to hide something)
Check for oil leaks near the dizzy on the back if the engine, put your fingers around underneath the fuel pump you'll feel a hole, if oil is coming out of there then it needs a new fuel pump about £100 for a genuine one but after market ones are cheaper.
Check around all the diff and transferbox flanges for oil leaks, could be caused by failing bearings. Check propshafts for play in the U-joints and slip yokes.
Most of all take it for a good drive, they don't handle like a sports car but they should drive straight and true with no wobbles, maybe a small amount of bump steer under heavy braking but overall it should handle well and be easy to drive. There should not be any loud whines (a few small ones maybe) or big clunks during gear changes etc.
As with all old cars just give it a good thorough checking over.
Hope that helps
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