RUST!!!
check for rust around the front and rear windows and check the pannel betwen the rear window and the boot, lift up the boot rubber seal and check ther also look in the boot under the carpet
check for box and diff noises as they are usually thrashed and you dont want to fork out for a rebuild
always good to do a compression test as the head gasket goes commonly also look for water in the oil.
Funny engine noises like bearings etc
Check for a smooth idle and if you have a multi meter check the cas and AFM
look for oil leaks and general stuff like that and if you can give the shaft of the turbo a wiggle and see if there is any shaftplay.
Check the shocks by bouncing the car with your own weight and should not continue to bounce more than a few time you apply weight, check for chopped springs and suspension bushes and make sure it isnt using oil
i am sure people will add and do a search, its been covered heaps