Why do you need a suppressor? It won't make the gun silent. Those make the gun longer, so harder maneuver. But they look cool. Unless it's one of those short ones that wouldn't even work on a pistol. Get ideas from real steel photos.
But what does help a lot? A red dot, a 1 point sling, mid-caps (+ mag pouches), Lipo + charger. If you play CQB, then a torch and a tracer unit.
I personally don't like the standard M4 grip, so I would switch that to a more ergonomic one.
Oh, almost forgot. At least 0.28g good quality BBs. Okay, if you spray and pray, then 0.25.
Edit: I checked that your gun has the tango down grip. That's a good one, keep that.