[hot_shot under fire]

In regards to optics, T1 Aimpoint all the way. Holds zero very well, doesn't short out from the recoil (Holo sights tend to do this very often) If you can invest in a real-steel optic then more power to you. Worth the investment in my opinion as you won't have to worry about it not functioning properly due to recoil or the elements. Just make sure you have a killflash to protect the lens!

As for the nozzle, I've found that one of two things will break. The front of the nozzle will snap somewhere, or the little spring that makes it return to its start position will break. I know Airsoft Parts offers the stock nozzle for about $25.00 each. I would advise picking a few up.

As advised above, stay away from the Aluminum CNCed nozzle by RA-Tech. Those tend to cause issues both with breaking other parts and very inconsistent FPS. However the Bolt Carrier by RA-Tech is a great replacement part if the stock bolt breaks.

Be sure to install an N-Pas, I know that most stock WE M4-platforms shoot a bit over 400fps stock. It's a great investment to make so you can play both outdoors and CQB, and it only costs about $20.00 plus it's easy to install and adjust.

Other then that, make sure to keep your gun CLEAN! Be sure to do a basic field strip and clean/lube the bolt/other moving parts every 300-ish rounds and/or before you put it away after game day. If the field is really dusty then clean it quickly between missions. I strongly recommend Frog Lube. It's about $15.00 for a little bottle of it and it will last a LONG time.

Aside from that, have fun! The world of GBBR's is a great one to be apart of!