I think for every INCH you take off, you lose 25 FPS on velocity starting at a 20" barrel.

As far as handguards go, thats something I think is preference.

What do you plan on mounting, how long, FF or non-FF.

There are literally hundreds of handguards out there. The style in the past 2 years alone is quite different as far as what people use.

Check out the AR pic thread. You will get a really good idea of how people mount different things and how they use them.