On the side: I'd highly recommend taking notes on fictional books you read like Keats has done in his margins. It helps to see how novels are all tied together like complex machines of tone, theme, foreshadowing, and payoff. It also helps to solidify just what the heck you're reading into words.
That said, I still think it deserves its place in world literature as one of the testaments to human creativity.
Well, it was written with the well-educated, versed in classics and latin and (then) contemporary social/political references, "Italian" audience of its time in mind...
I had a professor whose approach welded historical narratives into the works we were reading. In the absence of a good Medieval or English Literature Professor, I'd recommend something like The Connell Guide [1] or the Oxford guide [2]. If you're looking for an all in one with very good marginalia and footnotes, I found the latest Norton Anthology series to be very good (and usually considered the academic definitive). You'd need to find the volume that has Paradise Lost in it, however.
[1]: https://www.amazon.co.uk/Connell-Guide-Miltons-Paradise-Adva...
[2]: https://www.amazon.co.uk/Oxford-Student-Texts-Milton-Paradis...