To kind of repeat, and throw my 2 pence worth in, nothing is NEEDED, however I find a silencer (or tracer silencer if playing in the dark like I do) a front grip, Red dot and torch with a mount and a sling not only help me massively but look pretty cool too.
For me its important my gun looks pretty damn cool as well as functions well and these upgrades do that for me.
I have started out with cheap accessories, I will slowly upgrade over time, but a cheap silencer does the same job as an expensive one which is nothing, they just look the part. I spent a bit too much on my red dot but I am happy with it as it is not too big for my gun (MP5K). So just kind of go with what you feel you need. I bought mine bit by bit after skirmishes, I realised after one what would be useful for the next and got it accordingly.
Oh another thing not mentioned, A MAG POUCH.... yeah, its not a gun accessory but you need one none the less. Mags in pockets are a pain in the ass and are easily lost, trust me even a cheapo belt with a pouch is 100 times easier than filling pockets with bulky mags and losing them etc...