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I have a few WE M4s.
As others have stated, all you need to get going is an NPAS. If you have noticeable wobble between your upper and lower you can also get the RA-Tech "magic pin" which replaces your front body pin.
Eventually you may find thing you want to replace but you should use the gun first and figure it out for yourself. Just haphazardly doing mods will often create problems rather than fix any problem you didn't even have to begin with.
As for BBs, I recommend Madbull Match .25g, which have no/very few small air bubbles (the Madbull Precisions and most other BBs have larger and often uncentered air bubbles). The Match .25g will give you similar performance to other popular brands' .28s.
With the stock inner barrel you won't see any improvement in accuracy with heavier BBs: the only time you'd want to go heavier is outdoors if you need to combat wind and foliage.
Another cheap upgrade that does good is an improved hop rubber and/or spacer. This is a bit more esoteric and you'll find different people swear by different products/combinations.
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