Thinking of a custom predator/varmint AR in 6.5 Grendel

[Poor Knight]

I bought a couple of Anderson stripped lowers on sale from Brownells earlier this year, as well as some lower completion kits, in case Hillary got elected - hoping for a panic and was going to sell them for $$$$$. Since that didn't happen I am going to build these out and I am thinking of building a custom 6.5 Grendel predator/varmint hunting rifle. Here's my thoughts so far:

Proof Research 20 inch carbon fiber barrel with JP 6.5 bolt and firing pin (I emailed them and they said they are coming out with a 20-inch 6.5 Grendel barrel after SHOT)
Silencerco flash hider/Omega 30 suppressor attachment (already have this)
American Spirit Arms side-charge upper receiver
Superlative Arms rifle-length piston system
Lancer carbon-fiber handguard
Lancer carbon-fiber A1 length buttstock
Geissele hi-speed DMR trigger
JP silent capture spring
DPMS lower completion kit
ARG Target grip
Larue scope mount
Swaro z5 3.5-18x44 with BRX reticle

So my questions - the left hand side-charge upper is something I really want in this rifle, but would like some feedback on experiences with various uppers. I'm going carbon fiber to keep the weight down but any suggestions on lightweight handguards and/or buttstocks are welcome. I already have the scope and the Larue mount.

Biggest question is the piston gas system. I really like piston systems on my sbrs and carbines and overall prefer piston rifles to gas guns, but not sure I gain anything with this rifle length build.

I have the Geissele SD-E trigger on my other ARs and really like it so I may go with that - I like a flat trigger.

Any help and/or thoughts appreciated.

[Texas]

First of all welcome to the forum.
If you have built ARs regardless of the caliber, the build is the same.
The question about the piston or gas is a personal choice, but I tend to go with the military experience and prefer the impingement system to minimize moving parts and harmonics.
It will be interesting to see the final price tag on this one.
I am 90 miles north of you near the Red River.

[Lightning8]

I recenty built a similar 6.5G AR. I got a 20inch ARP barrel. I used the Lancer 15 inch CF handguard and the Lancer CF rifle buttstock. I used DI rifle gas system and "cheap" Anderson lower and upper. Works fine for me.