Stick with thr reputable brands, each will have its own "specialty"
how much do you want to spend is the real question
In an insane effort to double the cost of the original rifle....
my wish list (in partial execution) would be:
free float rail system - Yankee Hill Machine (just got one!) prolly the best value, Larue or Daniel Def have great rep though
tango down vert grip
low profile gas block - YHM again
Troy BUIS - Troy makes a great sight, but they are PROUD of it and the cost shows it
ACOG NSN - depends on what you want to do with it; aimpoint or trijicon dont go cheap if you put an optic on her
MIAD grip - MAGPUL
Magpul trigger guard - MAGPUL
CTR stock - MAGPUL - again, proud of these $$$ the 6 pos that is OEM will do for you for a while
IMO start with the hand guards (after getting a sh*%load of magazines), if nothing else upgrade to the M4 standard instead of the little spindly XM177 style they throw on the 15 ORs.
Guys will have their personal prefs and opinions, we tend to stay with what we are comfortable with. Just watch the cost, sometimes things gain a life of their own if some "expert" writes an article on it or it shows up on the cover of a gun mag. Take your time, the most time critical item you have (the rifle) so you are in the cat bird's seat.
Congrats on the purchase, enjoy!