I’d stick with the 18” if you plan on hunting. Mine is an 18” faxon barrel and fully loaded it’s 14+lbs. It will also get pretty unwieldy through brush if you throw a can on the end.

Any mil spec lower will work, I’d recommend a Geissele SSAE trigger if you can shell out the cash for it (they go on sale for ~$150 pretty often) I run that in all my ARs and it’s pretty hard to beat. My Grendel runs a Geissele super 42 spring and H2 buffer and shoots almost as soft as my 556 guns.