For accuracy start with a tightbore barrel.
For velocity I can't really say, it would be nice to know what gun you are using. If it's very high quality such as CA then you could but an m120 spring and then new aftermarked piston head. But if it's not then you'll need to start with a piston head and cylinder setup.
For ROF i'd use no more then m110 and highspeed gears, then after you have other things upgrades such as the piston then go with a systema magnum.
If you want all of the above be prepared to spend at least $250 in upgrades. This would include new piston, piston head, tightbore, motor, gears, etc.
And just a tip (Mods if this is considered "flaming in the newbie tank" then please send me a pm and i will know the boundries of what i can say), you will get MUCH better respones if you do your research and post a somewhat more knowledged thread outside newbie tank rather then us having to type it all out. The less work it is for us the more we willl help because we're lazy like that