Deleuze's Difference and Repetition can be considered hard to decipher, especially for layman. What prior knowledge and books are required/recommended before studying Difference and Repetition?
Deleuze is indulging in a traditional past-time of philosophy, metaphysics. So a first prerequisite is to understand what metaphysics is about. Traditionally, this is tied to theology. But what Deleuze is attempting to do is to found a secular - or rather an atheistic - metaphysics. So the second prerequisite is to have some sympathy for atheism, or even be an atheist - militant or lapsed. One way this is signalled, is that terms that only traditionally had a meaning within theology such as immanence or transcendence is stolen from theology to clothe this new kid on the block. Deleuze doesn't make any of this easy as he plunges the reader in his theory without much orientation. For example:
Everywhere couples and polarities presuppose bodies and networks, organised oppositions presuppose radiation in all directions. Stereoscopic images form no more than an even and flat opposition but they do depend upon on something quite different: an arrangement of coexistence, tired mobile planes, a 'disparateness' within an original depth. Everywhere the depth of difference is primary.
This is quoted in the review by Irfan Ajvazi mentioned by @Conifold in the comments above.
What does this mean? Well, Deleuze is marking out his territory and asserting his ambition of upsetting the natural order in Western metaphysics. The key, basic category in metaphysics is substance where sameness is the rule. Roughly, the opposite tack was taken in Eastern metaphysics, with the 'void' being the marker of the most basic category, particularly in Buddhist and Daoist metaphysics. Deleuze is more or less going from that angle, though he doesn't appear to notice this.
In the short extract above, "couples and polarities" refers to the contraries of traditional metaphysics that Aristotle affirmed that all philosophers assented to. This is also basically dualism of Daoism, it's Ying & Yang. Whilst "bodies" are the substances, again of traditional metaphysics. By terming it "bodies" Deleuze is also affirming his affinity with modernism which dispensed with the gods declaring bodies were all that is or was or will be: I am a body, you are a body, we are all bodies. Some extremely up-to-date people even go so far as to declare we are nothing other than our bodies, that is we do not possess minds - aka philosophical zombies. One day in the very near future, I expect them to declare we are no different to rocks. (Some already have, like the surrealists who confused themselves and others with chairs, hats, frogs, earth and the like. More sous-real than sur-real).
Whilst "difference" is his term for the void of Eastern metaphysics: substances are measured by what is, and asserts a difference. Subtracting substances places us in the void, and then there is no "is" but there remains "difference". Hence difference is "primary". By the way, his "everywhere couples and polarities" isn't a million miles away from pratiyasamutpada or dependent arising from Buddhist metaphysics. That Aristotle also affirmed this shows that perhaps traditional Western metaphysics is not a million miles away from Eastern metaphysics.
Thus as third prerequisite, it would be worthwhile to be acquainted with the canon of modernist writers and philosophers or those who were both. Particularly as he so often copies their style - abstruse & obscure - such as Pound, Joyce or Nietzsche. But all this will not make the going easy, as Irfan points out:
Any book as bold and influential as Difference & Repetition is going to develop a cult proclaiming its otherworldly perfection. Its every perceived flaw a secret source of wonder. I am not such a cultist. And there were a few things about Difference & Repetition that I found frustrating which I truly believe to be weaknesses of Deleuze rather on myself as a reader.
Simply as a well-ordered and coherent presentation of related ideas, his book is a train-wreck ... Deleuze seemingly cannot complete discussion of any aspect of his ... philosophy without switching to another, barely related branch of reasoning. Descriptions of key concepts are spread out, seemingly at random, in every section of the book ... indeed it seems to me impossible to have any understanding of it until one has read the whole thing and gone back and tried to put the broken parts ... into a systematic whole.
compounding the problem is Deleuze's insistence on renaming key concepts willy-nilly.
See above on "bodies", "polarities" and "difference"
I've heard Deleuze's most ardent supporters claim his refusal to adopt a consistent vocabulary is reflective of his conception of being as ever-changing. I'm going to call bullshit on that.
Well, exactly. Its hard to argue with that. Whilst it is true Deleuze is influential and important. Its just as true and important to recognise a similar critique was leveled at substance by thinkers in the East and from a much longer tradition rather than becoming a starry-eyed "cultist" drunk on the conceptual and literary feast that Deleuze provides.