I'd talk to some of your local high power folks before I spent anything.

I would go with Stag or Doublestar or Danial Defense in a lower. For a complete rifle, look to Bushmaster. All have good reputations. If you think you want Colt, make sure the takedown pins are the same size. I don't know about any S&W oddities.

I would look for a 6 position stock with any lower.

Uppers (think slide, barrel unit) can also be mail ordered (no FFL required) as a unit. So you can swap out an M4 carbine upper with integrel carry handle, 1:9 chrome lined barrel chambered in 5.56Nato for one with a longer bull air-guaged barrel, 1:12 rifling, flat top for optics and flip up open sights, high power 600 yard ready by pulling two pins.

Caution: Never run 5.56Nato in a rile chambered for .223 Rem. Overpressure!! But you can run .223Rem in a 5.56Nato chamber safely (just a little less accurate).
You can get barrels with a Wylde chamber that will be accurate and safe for 5.56Nato or .223Rem.

For 'yotes I'd run one of the 60gr or 65gr V-max cartridges or a similar weight softpoint.

A 1:9 twist is a good general purpose rate. Heavier bullets tend to become unstable or desintigrate in 1:7. 1:10+rates don't spin the under 70gr bullets fast enough to be accurate.

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John