Exactly how does one imitate a cajun accordian using a 30 buton CG anglo?
The nearest you can get is by playing it in octaves, in cross-keys on the C row, like a Cajun accordion.
... if I orderd a Cajun accordian, what key what I want to get that deep, rich, Cajun sound?
There has already been some discussion of keys for different kinds of music :
It is normal for Cajun fiddle players to tune their fiddles down a tone to play with the C accordion, but if the accordion player has a second instrument it is usually in D and the fiddle player will have a second instrument in normal tuning.
The commonest key for Cajun music is C, and that is the key all the instructional tapes, videos and workshops are in, so the best one to get to learn Cajun style.
However, if you want to play Irish or Quebecois music you will need a melodeon in D to play with other people.
Otherwise, D would be the second most popular accordion for Cajun playing.
Or you may have other criteria ? (They normally go as low as G and as high as F in pitch.)