A PDI spring would be better, they are a little bit shorter and they are offset so they don't put as much tension on your gearbox. They also will give you a consistent FPS for longer. I think systema is good too, but PDI is supposed to be the best.
And you said that you want ball bearing and metal bushings, you don't need both. The ball bearing are more likely to mess up with that FPS, but they would give you a higher ROF. and the metal bushings are just good because they are metal. (they both are metal, but you only need one or the other)
And I agree with Fish, you don't need a new piston because they are a lot better than people give credit to. Otherwise, looks good, I think I just ordered the same barrel.
