The best coffee to use is Medium Grind. Any brand is fine, ie Robert Harris, Atlas etc, all available from supermarkets
Work on the basis of 1x 250g bag per 30 people.
To fill the 100 cup urn, you will need 2-3 bags, depending how strong you like coffee.
You must use cold water to fill the urn, and it takes about 45 - 60 minutes to perc a full urn.