Battery tray area
Fuse box area
Back of sunroof on the roof
Under the sills
Rear arches
Tyre well in boot

Them are common places for rust.

As for other faults listen to bottom end rattling (good sign it needs a rebuild)

Bit of grey smoke on startup is your valve stem sills, cheap and easy to fix.

Blue smoke on hard acceleration is your turbo knackered.