I decided to build a Grendel after I was given a stainless steel 20inch 1in 8 twist 6.5 grendel barrel with a rifle length has system.

First, what buffer should I use with it?

It is a heavy barrel, so I need bipod suggestions as well.
I will probably be adding a six power scope as well.

The buffer is dependent on the buffer tube, rifle length buffer tube you need rifle length buffer spring and rifle buffer.   If you use a carbine length buffer tube use a carbine length spring and I’d start with the supplied carbine buffer, 3 ounces IIRC.

If your going to run a suppressor you might consider an adjustable gas block.   Many forum members like JP silent captured springs but I have never personally used a one.

Normally 20” 6.5 Grendel run smoothly and the gas is usually well balanced depending on if the correct gas port is used.

I used an adjustable gas block on my 20” Shilen 6.5 Grendel and I have not needed to adjust the gas since I first built my rifle.  I’m using a mil spec carbine length buffer tube, carbine length spring and supplied PSA carbine buffer.   I have three other 6.5 Grendel AR-15’s with fixed gas blocks and are gtg.   My 20” 6.5 Grendel was my first upper I assembled myself and if I were building it now I’d use a fixed gas block but because I live in Illinois and I can’t own a suppressor.

I have several Harris bipods and I think they're worth the price.   My Harris bipods attach to a sling swivel stud and I have a sling swivel stud on all my centerfire rifles.   I have a couple of off brand Chinese made bipods and they work but are not the quality of my Harris bipods.  I have a couple of 6"-9" prone/bench rest type bipods and a few 7"-12" bipods.   I don't have any of the taller height bipods as they are mostly used for hunting.  My bipods are not designed to swivel but for my type of shooting I never felt the need to use a bipod which swivels.

Each person's individual taste in bipods are different so its a good idea to try someone elses bipod before you buy.   I like using bipods which are easily attached and removed.  I have more rifles than bipods so I can easily use the same bipod on several different rifles.   I normally don't like the added weight if my shooting dictates I will not need the bipod.

I’m using a 6.5-20X50 Vortex Viper in Aadmount 20 MOA one piece mount with my 20” 6.5 Grendel AR-15.    A 20” 6.5 Grendel can shoot to 1000 yards so I like the higher power optic and canted mount.   I used the lower powered optics on my shorter 16” 6.5 Grendel AR-15’s.

OP use a higher power scope. Ive also got a 6.5-20 on my 20”. You won’t be able to get the maximum use out of the cartridge with just 6x.

I used a standard and heavy carbine buffer on my 20" rifle length gas system Grendel with no issues on either.  I also used a standard weight rifle buffer in the PRS that was on the rifle and again no issues. Mine doesn't have an adjustable gas block and has never needed one.

The JP Silent Capture Spring is a nice setup and is what I'm currently using in an XLR stock. Operates smooth and quiet.  It is the standard weight (no tungsten replacing standard weight rings).

I've used Harris bipods along with podlocks from T-Nutz. Both worked well. I prefer the swivel notched leg from Harris.  6-9 was just fine for my needs. Currently I have Atlas bipods on my precision rifles. I've been real happy with them.  The Harris legs are quicker to deploy,  something to keep in mind.

As suggested,  I'd probably go a little higher on the scope power.

I took 3 different 20" Grendels to the range last year around Christmas.

One was a Specialized Dynamics upper with Magpul PRS on the lower using standard rifle buffer and RET.

One was a Lilja AR740 build I did using carbine RET and a carbine 2.9oz buffer.

One was a Precision Firearms Criterion barrel using carbine RET and H3 buffer.

They all functioned fine for the limited number of rounds test-fired for accuracy.

The rifle RET seemed to run the smoothest.

The carbine buffer was a little more jumpy.

The H3 was better behaved, but maybe buffered too much.

I have a Faxon 20inch RLGS and I'm using a Spikes T2 with a springco enhanced power spring. That combo combined with a PWS FSC makes for a very soft flat shooting rifle.

I would personally go with a bit more magnification as well. I don't thing you need 20x necessarily and 10x is ideal IMO for the Grendel. Its really not hard to aim at a 1moa piece of steel at 1000yards with 10x for example.

OP I run a standard rifle buffer with a JP adjustable gas block on my 20" Grendel. My optic is a 4 X 16 but this rifle is heavy and doesn't get carried as much as the 18" Grendel. Are you going for a lighter weight hunting rig or extended range paper puncher?

If you're going for a hunting rig the Ace stock and a good 6X are hard to beat in the timber where shots are close to medium range. And you might consider two optics in Larue mounts until you build two uppers.

The buffer tube can be either carbine or rifle. An H buffer seems to work fine in mine, an H2 will increase felt recoil by quite a bit. I tried it and it wasn't good. With a 20" barrel you don't need an adjustable gas block but if you want one to fine tune it for a specific round then I highly recommend the JP rifle 2 piece adjustable with detent.

Use only E-Lander mags. Alexander Arms has a good supply of them right now. I use the 10 rd mags and put 5 rounds in them until they are broken in and then I can run 10.

If you want it as a hunting rifle then length of pull and cheek weld off the shoulder need to be considered. I run a Magpul PRS Gen3 on mine because I can adjust it and forget it and when I pull the rifle up I'm not wasting time searching for the correct eye relief. I've been having a heck of a time getting the PRS in black lately but they seem to be available in their earth color. There are others out there that are just as good.

The Grendel is not a Creed so optics for shooting out to 1000 yards aren't necessary. I have a 3x18x50 Leupold red dot on mine but I'm considering something like a good quality 3x9 for hunting to open up the field of view without having to do as much adjusting.

Out of curiosity, what's the reasoning behind this ?

Primarily to help you avoid the occasional issues experienced with some of the C-products, ASC or Stoner mags. Elanders are pretty much the gold standard for Grendel mags, these last couple of years.

The Stoner mags I got a few years ago are GTG.  Has quality fallen?

I've always suspected "AR-Stoner" mags were ASC with Stoner floor plates.

They are not. Not the ones I had. The Stoner feed lips are wrong and didn't have the little cutout on the front of the magazine like my ASC mags do.

The Stoners worked pretty well with wolf steel case but I constantly had feed issues with Hornady 123g ELD factory ammo.

I have Stoner, ASC and Elanders. Aside from the floor plates, my AR-Stoner and ASC mags are indistinguishable. They are also quite functional and reliable. As I recall, I bought them in 2012.

The Elanders have also been quite good and appear to be made a bit more robustly than either the Stoner or ASC. (Better welds, etc.)

The ASC and Stoner magazines I got years ago are and have been nice, functional and reliable. I don't have any E Landers so I can't comment on them.

There have been many variations and improvements on the CProducts and ASC magazines.

They continue to address how to make them better, just like with 5.56 mags.

The US military 5.56 NATO magazines are still in development.

I've been using light weight BCGs and adjustable gas blocks in all of my ARs these days. I combine it with the lightest buffer I can find (I just got a carbine weight Armaspec SRS to try) and then dial in the gas to match the setup. You can make some pretty soft shooting guns that way.