If you're going to be learning off of instructional dvd's, I think you should go with a C. All instructional's I know of use a C accordion, and most recordings.

To paraphrase M Savoy..

Get an accordion in C.. if in doubt, get an accordion in C.

I have an accordion in C.. just in case..
but 90% of the time I play on a D box.

Savoy and I agree on one thing (seems to be the only thing)
I much prefer a box in Bb or Eb.

I also like A boxes.

In preference, I'd rather a D. But for someone learning Cajun music, especially off of dvd's, and if the potential is there to go to jams and instructional camps, you need a C. Once you get good at taking a tune and converting it from your ears to your fingers, it doesn't matter what key except for what you sing in, if you sing, or who you will play with, if you will play with someone els.