I have a slick side upper that has been my hunting rifle for a good while. The lower has a CMC flat trigger and good parts. I want to build a Grendel so rather than buy an upper with mediocre barrel, I want a good barrel and build on my slick upper.

A friend has a new "20 Ballistic Advantange type 2 rifle gas barrel I can get (opinions?) and I will get a good bolt to go in my carrier.

Now, do I need an adjustable block or not?

Will my current carbine buffer and spring be fine?

Should I have the upper's barrel face trued?

Join the 65 Grendel forum. Buy one of their group buy barrels and bolts.

ETA
Grendel Forum

I’ve tried many different Grendel barrels. Buy the best one you can. If buying used, I’d want to see groups and know what ammo shot it.

Most of the specs that get communicated are velocities with a 24” barrel and ballistics using a Lapua or other match bullet.

I don't know anything about the BA barrel in a Grendel but my experience with them in other calibers is they are an ok rack grade barrel.  If you want a great barrel look at Lilja, Proof Research, LaRue, etc.  The Grendel Forums has barrel group buys going on for an excellent price but I have no idea about their quality.  Whatever you get stick with a SAAMI chamber and .136" depth bolt.  I've used the Maxim bolts and they are great.

Grendel is a lower pressure cartridge, ~ 50,000 psi and usually well behaved.  They can't hurt but unless you plan on running a suppressor an adjustable gas block is probably not required.  This assumes the gas port is drilled proper size.  None of my Grendels has one.

I run standard buffer and springs in my Grendels.

Lapping the receiver can't hurt.  I use blue Permatex gel to "bed" the barrel.

I also recommend joining the Grendel forums.  Get the Grendel Reloading Handbooks, both volumes.  Even if you don't reload lots of good information on the Grendel in them.

Grendel is a mild round. I have a 20" and had an 18". Never had an adjustable gas block on either and functioned both suppressed and not. Flash hider or brake didn't make much difference.

As far as lapping the receivers,  I haven't done that on any of the AR I've had and accuracy has always been very good.  This includes the Grendel's and the Creedmoor's I currently have.

Stocks I've run A2 and flavors including PRS's. Standard tubes and buffers worked fine. In carbine stocks I've used ACS, ACS-L, STR, CTR, UBR and standard bodies. Standard tubes and buffers worked fine here as well. Currently,  most of mine have XLR stocks and JP SCS's in heavy configuration but with the Grendel standard configuration worked as well.

Grendel is not real picky on ammo,  but cheap ammo doesn't group worth a damn. Good grade ammo does far better and tuned handloads are excellent.

If the barrel has been taken care of,  is from a decent company and hasn't had an inordinate amount rounds run through it, it may be worth looking into.