The list of things to check is a long one but I'll put everything I can think of and hopfully people will add to it.
1. Body, open the boot and look at the rubber grommets where the wires come out of the car and into the  tailgate, they usually have rust around these areas on the tailgate and the car body itself. Then have a good look at the tailgate as there known for rusting behind the number plate so give that a close inspection and check the drain holes at the bottom of the tailgate, have a look under the carpet in the boot to see if the back lights are leeking and have a good look at the boot floor for signs of rust or repair of damage. You didn't say if it was a 3 or 5 door so on a 3 door check the rubber around the back passenger windows as they can start to go rusty around the bottom edge of the rubber. On a 5 door open the back doors and you should see a bolt close to where the door lock is located they rust around this bolt and on all doors check the bottom for rust, check that the drain holes are not blocked  or rusted out. Open the sunroof back as far as it will go and check the condition of the sunroof visor and arms as both of these can be very rusty and may need replacing in order to close the sunroof properly without having to push the visor down by hand, check the condition of the seel to make sure there's no splits or cracks in it and check to make sure the drain holes aren't blocked or rusty. Also have a look at the guides to see if they've been lubricated, when you close the roof it should close smoothly and not stick or jerk and then check the condition round the edge of the panel for signs of rust. Have a good look around the windscreen rubber as these can leek and if the problem is not put rite the water will make its way down to the fuse box. Have a good look at the panel just below the windscreen as these can also rust and have a really good check around the wiper spindles and especially around the blanked off hole on the passenger's side. Check the battery tray, these tend to rust on most older cars and can be fixed but you don't want one thats too bad. Behind the exhaust manifold there should be a heat shield that runs down the bulk head and under the car check this shield for rust because if its very rusty or not even fitted the bulk head may be rusting as a result of being exposed to the heat of the exhaust. Check the front wings, bonnet and front panel for signs of accident repair work and if you can have a look under the bumpers at the front and rear valances. If the car is or has been changed to a big bumper get on the floor and look up under these as they are sometimes fitted to hide accident damage. Open the fuel flap and check that the drain hole at the bottom is not blocked and that the area around the filler cap is not rusty. General checks should be to look at all the panel gaps to see if they are equal all the way around the car, check around the door handles and if it has armour plates fitted are they there as protection from a break in or are they covering up damage caused by a break in ?
This is just a small list of things to be on the lookout for and at the moment I haven't got time to add any more but if anyone wishes to add anything I've missed or correct me if I've made any errors please feel free.
