Unfortunately, we really don't have any built-in way to accomplish quite what you describe. Selecting a collection of files to purchase would normally be a matter of the buyer simply adding their preferred items to their cart, but we have no way of requiring a minimum number of items before enabling checkout.
You might consider setting up single-file products with a fairly exorbitant individual price, so the buyer would have incentive to add at least 6 items to qualify for the discounted prices you described. Then you'd set up a series of discounts for All Cart Items that take increasing percentages off the buyer's cart when Min. Items in Cart = 6, 8, and 10 -- e.g., if you set the prices of individual items at 10.00 each, you'd set up one discount taking 55% off the cart item total when Min. Items = 6, then another discount taking 57.5% off when Min. Items = 8, then another taking 60% off when Min. Items = 10.
You can also use our cart customization code to add some custom text/HTML to the cart explaining this: