dont buy chinese bits.
uppers, pretty much all the same, never seen an upper fail so buy whatever you want really
Lowers, dont buy a carbon fibre one, seen one of those pop, not pretty, try to look for american brands like, DPMS, RRA,Colt etc
Barrels, i have a mil spec barrel in one upper, and its ok for hunting as its light, but if you pop off a few rounds it gets hot and starts spraying, have used MAB in the past and was fantastic, and my new uppers are being built with trueflight barrels, which i am sure will be fine
forend - personal choice here, $50-100 for a cheap free float tube of $300+ for a fancy railed one, just dont buy airsoft rubbish
Grips and stock, A2 rear stock is cheap and works well, again you can splash out and buy something fancy later if you want to. any pistol grip will do as long as its not a mil spec one, again pick something that works to start with and you can always change it later
trigger, get something nice, Jard, RRA 2 stage, national match, ar gold, etc. you wont regret spending more on your trigger.
bolt and carrier, just buy something from a know manufacture, DPMS, RRA etc
try the AR builder on brownells