To have a decent look your gonna have to remove the sill cover and maybe the wheel arch trim, these are only retained by pop rivets so its no hassle to get new ones to fit it back on, of course still available at your friendly main stealer.
I would say its still quite rare for a golf to rust badly unless its had damage before that has either not been repaired at all or has been repaired badly.
Sounds to me like its had a repair and hasnt been seam sealed properly. I understand about the plastic sills being a water trap but it still sounds like its had a shunt in the past.