Nick,What kind of rifle are you wanting to build?
What type of distance are you wanting to shoot?
Are you wanting a lightweight / mid weight / or heavy barreled rifle? I personally like 18 or 20 inch barrels,and use a rifle length gas system,but I shoot for distance and accuracy,and also only handload for all of my firearms.If you are planning on shooting factory ammo,the rifle will only be as accurate as the ammo that you are shooting.
You can buy stripped uppers and lowers pretty cheap right now,but to build a quality AR for $750 will be stretching things real thin.
The lower if bought from an online vendor will have to be shipped to a local FFL,and then they will charge you a transfer fee on top of the price of the lower,so if you can find a quality stripped lower for $100-125 buy it locally.
I've bought several upper's/lower's from Surplus Ammo,and they are good quality. http://www.surplusammo.com/saa-sa-15-reticle-logo-ar15-stripped-lower-receiver/
I use Rock River Arms 2 stage NM triggers in all of my AR15's/AR10's,for the money,they are one of the best triggers around.There are better triggers,but you'll pay 2-3 times more for them.
You can buy the RRA trigger on Ebay for $80,most places get $100+ for them. http://www.ebay.com/itm/Rock-River-Arms-National-Match-2-Stage-Trigger-/191474315744?pt=LH_DefaultDomain_0&hash=item2c94c2 31e0
Barrels are where you get into the money.A good barrel will run you $200-500,but you can get a really good barrel for around $250-300.
Rainier / Seekins / White Oak / Wilson Combat and several others make real good barrels.
Bolt Carrier Group will run around $100 for a good one,then you will need a gas block/gas tube/handguard/buffer tube-buffer-and spring/and a buttstock.The handguard and buttstock can also get pretty expensive depending on the style that you want.
Sights/Optics - Here's where the big money can be spent,but there are deals out there depending on what you want.
I'm not trying to discourage you,but you can sink a ton of money into a quality AR,you can do a little at a time,or you can buy a cheap rifle and deal with the performance.