It's a bit awkward.... I think better: Everything you need to know is here. now, regarding the use of "it"...you don't need the "it" . "Here is what you need to know now and whatever you may need to know later." but again...awkward.

The first instance is not needed and reads more "natural" without it, but the second one is obligatory, i.e.: "Here is written all you need to know for now, and also whatever it is (that) you may need to know later on". Just to add an alternative on how it could be worded if I were writing a list of instructions: "Here are a few things you would need to know for now, along with some others that you may need to know later on in case something comes up." {It sounds a little presumptious to say that you have thought of and provided every instruction for every conceivable scenario, so it sounds more "humble" to just say say "some others" or "a few others".

it is called expletive or pleonastic "it" because it does not carry any argumenthood - same as a swear word.

'It' is a pronoun. An expletive is a swear word.

I just made up this sentence to try and work out a way to show the expletive "it". It might not be the best sentence in the world after all. Hopefully, it is grammatically correct, though a bit akward.