Couple of things you might wanna look out for...
1) Water/petrol can collect under the arch trims and underseal and leave you with rust problems. Keep a good eye out for this.
2) Also check the runners above the rear lights as dirt can collect, block the runners and cause rust too. These problems seem to be less severe with metallic cars.
3) Check where the petrol filler neck connects to the tank as they rust through.
4) Check the back of the battery for rust
5) Check round the window rubbers for signs of a respray
6) check in the boot for the boot sticker and to see if the boot floor has any bends/ripples in it (signs of accident damage).
7) Check the front wings for the VW logo and clips each side

Check the bonnet for the rubber buffers when it's closed