What are the dimensions of your comforter? Is the fabric washable, and would it shrink? You'd have to allow a few inches for that. The salesperson you buy from should be able to help you calculate how much extra to buy for pattern matching. It will depend on the size of the pattern and the print repeat.
I have a queen sized comforter(84"x84")and would use 5 yards of fabric for one side for most fabric widths. The duvet top would be three pieces sewn together...one wide center piece with two narrower strips on the sides.
You could use a flat sheet for your bottom piece if you can find the right color. HTH

I have a queen size comforter at 90 by 90 inches, and while I make my duvet covers out of sheets, my finished measurements are 88 by 88. Measure your comforter first take off and inch or two and then calculate what you'll need, including shrinkage. Using sheets makes for a lot less work. As the other poster said it would be good to use for the bottom piece.