Everything.

Strut tops, (front and rear)
Slam panel,
Arches front and rear under the bodykit
Inner wings (both from the engine bay and the wheelarches),
Battery tray
Fusebox
Inner cills, if you can peel the carpet back
Check the seats are sturdy cos the mounts rot
boot floor and spare wheel well
any rust round the sunroof is very bad news, even bubbling usually means alot of hard work to fix
Tailgate hinges

Engine, check for smoke on idle or when you back off the revs this could be the stem seals gone, or the turbo in trouble.
blue smoke is the rings.
all the usual engine stuff, smooth idle, no excessive smoke, no whiney gears etc.

There really is so much you could write an essay and still miss something!