S&W M&P AR in .300 Whisper (factory rifle)

[Romeo]

Hello, I was hoping for some help.
I've been reloading over 20 years. Cast my own heads, etc. I have an old friend who has 2 SSK 300Whisper T/C barrels.That's how I originally was exposed to the Whispers.
My Q. I've been looking at the AR uppers available. SSK is a little out of my price range. Legally I need a 16" barrel. I would like an upper which would shoot and cycle w/ super and sub-sonic loads. Would I need an adjustable gas block to do this? The only SSK Ar upper I've seen (16") had an adjustment screw and block on the center of the gas tube.
I've been leaning toward the S&W upper. Hoping you can guide me in the right direction.
I have Foster neck trimmer/ reamers etc. Plan on using Mil. once fired LC brass. I saw HOSER's thread on brass.
The rifle would be used mostly for paper/ hunting, and sub loads for pests. One of my favorite past times is building accurate loads for a particular gun. Also, What should I get for dies? There are a lot out there. I usually go w/ RCBS but I;ve been looking at the Foster and Reddings.

Thanks for the help, Jason