Hi,
Indeed, I would recommend using a characteristic with 2 variants in your case.
In the downloadable variant, you can add the file in the Images & files area so that it would be available to the customer after the purchase.
And in the physical variant, you could use the "quantity per order" setting to set a minimum quantity. And you can also configure several prices with different "minimum quantity" in the variant. So that you can have discounted prices for bulk purchases.
I don't think the "discount" feature we provide in the Orders>Discounts menu is adapted to what you want to do since you can't restrict a discount to a quantity being purchased. A better option would be to have several prices, as I said above, or you could also configure a "coupon" ( in Orders>Coupons) limited to that variant, with the "auto load" setting activated, and the minimum / maximum quantity settings. But with a coupon you would only get the discount applied during the checkout, and you can only have one auto loaded coupon per cart. So I think configuring several prices in the physical variant would be more suited to the situation.