I think the approach of having users select the sets of cards will be the easiest to implement. When it comes to the discount for the quantity there is a condition that you can use for any column value. In the eCart object in the discounts when adding a trigger you should select the Number of items with a specific column value, then specify the category column and the criteria for the discount from there. I think this will yield the result you are looking for and be a little easier to get into place. Post back if you have any other questions or ideas on this.