Faulkner had a strong background in traditional narrative and poetry from which he was encouraged to expand by Sherwood Anderson. With his background, Faulkner would have been at least familiar with Paradise Lost and would have been concerned with the same themes as Milton was, though Faulkner talked about "eternal verities" of truth and virtue. However, critics generally only name Sherwood Anderson, Mark Twain, Charles Baudelaire, A. E. Housman, among a few others, as influences on Faulkner.
All of Faulkner's works were influenced by British authors, as most American authors were. Conrad is one example, but I think writers like Dickens and Austen should be considered foundationally. They both wote about the human condition, to get people to think about the world around them.
I think the characterisation of Sutpen is something that you can gain a lot of mileage from, especially in the way that he could be viewed as relating to Satan and Kurtz. You might like to think about what other parallels we can draw between these texts. Does Sutpen have a doppleganger, like Kurtz does in Marlow? How does Sutpen relate to the other characters in the novel, as Satan relates to Adam and Eve?
It looks as if there's an article by Stephen M. Ross that may help you make your case for the influence of Conrad (see link below). Surprisingly, the overview of scholarship linked below does not discuss many other "source studies," but this survey is several decades old at this point. You may want to go to Google Books and search for "sources of Absalom, Absalom. Good luck!
Here are some leads on a possible connection between the novel and Paradise Lost: