Individually priced files are not what they seem to be. In general, they are not a good option for anything. The only clean option they give, is rewarding buyers on a 0 cost pay what you want game. Other than that they are very inflexible. Want to increase price later? You can't. The individual part means only that the buying price of a project is compared to the "individual" price of the file - at the time of download. This practically means, that the buyer did not buy anything. And if you do things like raising the minimum price above the individual price or increase the indiviudal price, you screw over all buyers that "purchased" it at that price. It is a horrible pricing option for a store. (Did I mention that you cannot increase your payment tier? Pay 5 and pay 5 again and your tier is still 5 and not 10, but now you have the game twice in your library. So it is an utterly useless and misleading option to offer a game for 5 and have an individual priced dlc file at 10.)
There is no conditional projects as far as I know. You can have conditional sales. That is, offer a project for reduced price, if a specific project is owned by the buyer.
The cleanest way of having dlc is seperate projects. And if you want to avoid clutter, you can have those projects only be linked by the main page, instead of trying to have them appear as regular projects. You can bundle projects and bundles can have a separate discount.