It should be noted that even Kant said that Hume "awoke me [kant] from my dogmatic slumber." Hume is pivotal to Kant's work, so at least reading the Enquiry is a good thing to read before Kant.
While true, and I realize the irony of saying this since I mentioned building up to Foucault, I don't think one is harmed so much by simply starting. There's a lot of people who try to pile on more and more prerequisites and I think it's less productive; for every person who tells you to read Hume before Kant there are ten who will tell you to read Berkely before Hume. Personally I simply entered what interested me; first Marx (which is an ongoing love) then Hegel and now Kant. Heraclitus can come later.
