WOW...lots of stuff there....
Short answer: Buy a G&P Complete M120 mechbox...reshim, swap spring. Swapping the piston is optional. (you can paint it pink if you need to
). You'll save yourself a heap of time and headaches with the little springs, screws and other bits and pieces. And you'll have a working baseline to compare the performance of each of your upgrade parts against.
Long answer:
Prometheus or PPM (might be PPA...but they're the same Prometheus parts before they're branded Prometheus) pistons are nice. Stay away from aluminum pistons and aluminum piston heads.
I personally have always liked the Systema Silen Head sets...but a lot of guys seem to go for the Sorbothane stuff.
I've used aluminum, brass, SS and teflon cylinders....personally, the plain old brass ones work just fine (actually, the aluminum one worked the best and is still going strong). Not as flashy as the SS or Systema Teflon ones...
No need for a bore up kit in that setup....but watch your cylinder porting.
Modify Airseal nozzle is fine. Modify bearing spring guide is fine. I have lots of setups that have both a bearing spring guide and a bearing piston head. Use Modify springs and there'll be no issues with the spring being too long. Modify tappets are fine. Selector plates have little impact on performance...so KA switches do not have exposed contact fingers.
14AWG is a very tight fit...16AWG is still a tight fit especially if the insulation is thick...18AWG seems to fit no prob. DEANs connectors are great.
Sector chips/timing chips help by holding the nozzle back a little longer per shot to allow mags to feed bbs in high ROF setups. (I think the G&P complete mechbox comes with one).
With AEGs...the "trigger lag" and ROF go hand in hand. With a quick trigger response (i.e. from when you complete the trigger pull to when it shoots) you'll end up with a high ROF.
Use a large 7.2 or 8.4 NiCad/NiMh battery...that may give you what you're looking for...but not so good for a stock tube setup. A 7.4 LiPo "sounds/feels" like a large 8.4v or a 9.6v mini. The long or short body stub doesn't matter (i.e. just cut a long one shorter...)...the limiting factor is the stopper inside the stock tube. KA makes one that is "deeper"...but get in line, the next one is mine.
TM EG700 or 1000 will be fine. Guarder Infinite Torque Up are great for the price. G&P M120 are good. Systema Magnums are great but expensive. If I was scratch building I'd put a Guarder in it.
The Magpul MOE grip is very nice....it's not going to impact the noise level...the most noise will come from the motor pinion to gear setup/adjustment....and the rest of the gear set.
Best of luck, hope it turns out well.
Tys