New guy looking for advice on his first build

[Gator]

Youre pretty much GTG fromwhat I see on your list. As for the hammer forged barrel, it's subjective. from a practical standpoint, are you going to be shooting quite a bit? classes and all? Would you realistically benefit from its features? If so then it may be the route to go but then again, the standard ones are GTG too for the above applications.
Rail length will depend on shooting style, IMHO, placement preferences of lights, grips etc... Some need it, and some dont necessarily. Since you already have a low pro GB, I would suggest going no shorter than a 10". That way you'll have more room to find what works for you and also give you a longer sight radius.
If I may ask, why that stock though? No experience with it personally so I cant say it is or isnt good but since this is your first build, I would get a good adjustable stock. That way you have 5 - 6 positions to choose from to find what works for you.
Judging from your current choice, may I assume want something slim and solid? The Magpul CTR should work perfectly. Slim and locks up solid with its friction lock feature. Also comes a rubber butt pad which I also see on your original option.
Vltor also offers very good stocks--ones what would allow for both a slim or wider cheek weld config.