Most important thing to remember when looking at a p6 is that the sill you can see is simply a bolt on cover, the real sill is behind it, which makes it very difficult to assess the real condition.
Also as all the exterior panels bolt off easily, it is easy to make a structurally weak car look perfect, appart from checking for obvious rot and panel gaps (never perfect) don't give the exterior panels too much time.
Important areas to check are the D post, open the rear door and take a look, try and work out if its been filled ! Also check the inner rear arch behind the D post, and also the bottom of the inner arch.
while you've got the back door open, lift the rear seat base (ask nicely ! ), there should be a load of sound deadening under there, lift this out and you are basically looking at the inside of the floor, it is very common to get rot in this area where the inner sill meets the floor.
Check all along the inner sill where it meets the floor, signs of rot here usually indicate much worse rot on the hidden out sill.
check under the front wings, make sure the full structures are there, its common for "hockey sticks" to have rotted away, these are stiffeners under the top of the inner wing, where the wing meets the engine bay.
Open the bonnet and check the areas where the wings meet the engine bay, quite common to get rot all down this edge especially once the hockey sticks mentioned above start to rot.
Other than that, its the normal engine, brakes, steering stuff, drive it, if it feels right then it probably is.
There are probably a few more things to add, others I'm sure will have their "pet rot areas" !
Hope I haven't frightened you off !, but its important that you don't pay over the odds for a car that may require new sills and half floors on both sides !
Richard