I don't have my Owner's Manual with me at work here, but look into the section "Maintenance" in yours, and you should find the correct amount of brake fluid which is in your system. Haynes might also have that documented.
At the worst, it is unusual that you would have to go through more than two cans of brake fluid, assuming the 12 oz. cans; you might consider going with the ATE Gold and ATE Blue Brake Fluids, since that way you can alternate them between fluid changes, and you will see immediately when the "old" color comes out of the caliper. The next change, use the other color, and repeat the process.
Many German-car dealers sell them, and Zims in Texas advertises them for US$ 8.95 for a pint, which is a lot more than 12 oz. Telephone number is 1.800.356.2964...Andras