Check the door bottoms to ensure the drain holes have not become blocked and caused a rusty/crusty mess...!
Open up that bonnet and check the front scuttle area (where the windscreen washer jets are) as that little 'ditch' can be neglected and get filled with soggy old leaves and dirt thus causing the onset of our good old rusty enemy to start attacking...!!!
All I would recommend is be thorough and check (then double check!) everything you possibly can...! Check that all the electrics work properly, check that all the mechanical bits do everything they should. Then when driving it ensure that it 'feels' right and it travelling straight and true and is not making any weird clonks/rattles or squeaks (although they all do that a little bit!)...
As well as the body ensure you check the interior to be sure it is a comfortable, habitable place to be... You need to budget for work in that area if things are looking a little tired/tatty!
The more documentation the car has with it the better (especially more recent stuff) as you can see what has been done and then see what may need doing in the near future (this can be a good bargaining tool!)...
All I can say is be careful, try and not get over-excited (which is what I do!) and be as overly critical as you can... Remember you are buying a car and it is the sellers job to make you WANT to buy it so don't feel forced in to something you're not too sure of...
Now just go for it, good luck...