For Paradise Lost, and for just reading you can go with almost any of the "commercial" versions: Oxford World Classics, Penguin, Norton, or Modern Library's Complete Poetry and Essential Prose of John Milton. If you want a scholarly version that really goes into explanatory detail, the Longman Annotated English Poets is phenomenal. All of them will explain the difficult parts of the text well enough for you to understand it, and they only differ in the depth of those explanations. Oxford, Penguin, and Norton have the least detail, but Norton does have a lot of supplements that are helpful. The Longman has, by far, the most explanatory (scholarly level) detail, but it's probably too much if you just want to read for fun. The Modern Library version is somewhere in the middle and it's actually the version I first read Milton in many years ago. I would also definitely recommend picking up a version with Dore's illustrations, but I'd probably recommend reading another version first (perhaps use the Dore afterward to read along to an audiobook version).
As for the difficulty Milton is on par with Shakespeare; it's early modern English so the language has changed some since then, but not as much as with, eg, Chaucer. Perhaps more difficult than the meaning of the language is Milton's "grand style." He basically tried to imitate the classics (Homer, Virgil, and Dante; but especially Virgil) in English blank verse. Before Milton, blank verse was mostly the poetic form of drama (like Shakespeare), and was used for its flexibility in being somewhere between poetry and prose/regular speech. Milton radically changed that through both diction and syntax to mirror the heightened, epic, "sublime" quality of his predecessors. Reading Milton was the first time I thought to myself "I didn't know language could DO that!" I will say that part of the fun of Milton is listening to and enjoying the language, apart from just what the language means.
For The Divine Comedy I prefer the Ciardi translation for just reading, and his notes are adequate for explaining the necessary contexts and references. For explanations, it's hard to beat the Hollander's. People do differ in what kinds of translations they prefer, whether they want something more literal or more poetic, more "updated" or more archaic (closer to the original style/language). The Ciardi is more on the poetic side, and he's one of the few to maintain Dante's terza rima form, which inevitably means he must occasionally divert from the original's literal meaning to make the rhymes/lines work. The Hollander's are much more literal; they maintain the verse line, but there's no meter or rhyme, which (IMHO) ruins much of the effect of reading poetry in the first place. However, their explanatory notes are the best I've come across.