Depends what you're after I guess.
If your buying a calais, it already has a subtle body kit on it anyway as it's designed to be a car of elegance and refinement more so then something with a massive body kit on it.
The calais is quite an an attactive car stock. All I would do is put 18" mags of some kind on it (VZ SS or VZ CV8 Monaro), lower it and tint windows. Oh I would change the exhaust too if it was a v8
I have a VY Series II Berlina which I have done similar things to. I chose a berlina since I didn't want leather trim.