@OP,
I shoot ARs in competition a lot. I use to shoot a Colt HBAR in Service Rifle competitions before I became disabled. I now shoot a home built (more like assembled) AR for use in F T/R matches, ranging from 600 to 1000 yards.
Please allow me to address your questions.
1. Re: twist ... this will influence how far you want to shoot accurately. [Re: accuracy, I am looking at it as a target shooter striving for 1.5 MOA as a service rifle, about 0.6 MOA for a match rifle.]
1a. With a 1:9 twist, the heaviest bullet you can launch accurately would be close to a 72gr, out to about 500 yards, maybe 600 on a really good wind free day. Expect that you could launch a 55gr bullet accurately to about 300 yards, but pray if you have much wind.
1b. With a 1:8 twist, you can launch an 80-85gr bullet accurately to about a thousand yards, provided that you keep the bullet supersonic at the target. It should launch a 55gr bullet accurately to about 300 yards, also. On paper, I have a working thousand yard load, but it peters out by 1017 yards ... it is hard to test, since my home range is 1039 yards.
2. Re: plastic or nylon handguard. The handguard does much more than keeping your hand from getting burned by a hot barrel! Not all handguards are free floating. The handguard is under considerable stress from the torque inflicted by the sling, when you use a sling. Personally, I would recommend that you look at a rifle with a metal (aluminum or steel) handguard, which does free float the barrel.
As an aside. Unless you are equipping your entire family, you can get away with a single lower, with any variety of uppers. I have 3 interchangeable uppers for my "AR", a 26" 1:8 heavy barrel that is free floated for F class matches, a 24" 1:8 heavy varmint barreled upper that was going to be my "spacegun" match rifle before I became disabled, and an 18" 1:8 heavy barrel for a CQB rifle. All of my rifles are mission specific, and in this case, an AR platform can be adapted to a specific mission in under 3 minutes! In some instances, you can build an upper for under $125. Consider purchasing an AR with a quality lower, and run from there. A 1:8 twist can be more versatile and not limit your ammo selection. You can also change cartridges within "family", just by changing the upper.