Honestly I would hold off and get a different tang instead. There are several tangs that are just as attrative as an Achllies but not nearly as hard to keep. I, like you, wanted one and a friend of mine tore down his 200 gallon and gave me his old one. They first off dont ship well at all. They stress very easily and often wont start eating, in addition to that it seems as though other fish can sense thier nervousness and pick on them worsening the stress. In addition to that these are fish that inhabit the surge zone of hawaiian reefs and these zones, that contain numberous bombardments of indirect flow are almost impossible to re-create in the aquarium.
To re-create a surge zone you would possibly risk killing everything in your tank. A friend of mine who manages a fish store has a 500 gallon tank that once housed an achllies tang. The tang, even though the tank was huge, never got used to captive life and ended up ramming into the glass one day killing itself. After keeping one and seeing thier captive state, and researching this fish rather in depth I think you would be better off with something else even in that large of a tank. If you like large tangs look for a Red Sea Sailfin Tang. These are marvelous fish that get large and have incredible markings.