To get 350fps you need a spring between a m100 and m110 which will depend on how good everything else is in your aeg.
Accuracy, range and groupings is all about barrel, hop and ammo.
Firefly bucking, prommy blue flat nub and a PDI 6.08 barrel is probably the easiest good set up. Use 0.28g bbs and it should be good if set up right.