Still learning about AR's. I tried googling alot of those parts to see the difference.
0.This deal includes the upper and BCG?
The page does not state if its included I'd give them a call they're good about returning calls.
1.Is there a difference WRT the rails?
Different makers/length/apperance/free float or not
2.Of those that are listed, is there a brand that is better quality?
No opinion YMMV. I went with the DD Omega rail based on the reccomendation of a friend.
I have not been dissapointed.
3.When choosing rail length for a 16in barrel, is it all preference?
Depends on the gas block/sight post, length of gas system and the number of doo-dads you want to hang.
4.If I wanted to install the Aimpoint PRO and Troy flip-ups, seems like I would want the low profile gas block, yes? (just to be sure)
Yes.
5.And lastly, the comps/hiders. Don't really need anything fancy, mainly just MA legal until I move to NH. Standard A2?
NO. Standard A2 is a "flash Hider, you'll need a comp- pinned & welded for Ma.