Make sure you see it started from cold. No rattles or smoke as stated above. Get a look under the oil filler cap, no mayo and hopefully no excess sludge visible.
Check if any meaningful service or repair work has been done; clutch, flywheel, rear beam bushes etc.
Take a close look around arches and tailgate handle for any rot.
Check central locking and correct function of interior lights when open doors and boot.
Make sure all dash lights go on initially and off after startup.
Take it for a spin if you can. No clunks over bumps.
If you have access to a code scanner and owner doesn't mind plug it in and look for faults.
Happy shopping!