Like any of the add on guns-
AR, 1911, 10/22.
I think the buy Vs. build is all what you want on the gun.
Personally If I have to change more than 20% of the gun I build. Especially when you are talking major componets like the Upper, barrels, or slides.
Funrniture, Sights, and other bolt on parts start out with a stock gu as close to what you want and then Bolt on the parts you want to change the gun to your tastes or needs.
I think Jamayton's picture is the best option, Extends the forend to what you want, allows you to keep the barrel in place and atach all kinds of gizmos to the rifle. Including iron sights to replace the front sight if you removed it or replaced it with a gas block. I like Iron sights on the AR- All battle rifles need Iron sights.
On the other side of the coin if you are looking to build up a hunting rifle, swaping out the barrel to a 6.5, 6.8, or 458 may be the easy way to go, since free floating the barrel will require you to remove the barrel. New bolt in the case of the 6.5 or the 6.8, Heck you can even go 6X45 or 7 TCU (223 necked up to either 6 or 7mm) for hunting deer and pigs.