If you think you are going to build start shopping for a stripped upper and lower NOW. With the upcoming election finding the ones you want could be tough. Given your budget you'll be looking at forged parts for the upper and lower and both are very difficult to find right now. The cost for both will be as little as $120 ($60 each) depending what you chose so it's not a big expense to open your wallet for ahead of schedule.
As for where to start buying there are tons of options. Places like Primary Arms and Palmetto State Armory are more AR oriented. But there's tons of small shops that make various parts.
Brownells has a PDF "parts list" that is supposed to help you figure out what you need but I found it terribly confusing.
For the lower you will be buying some sort of lower parts kit. They come in various levels of completeness. If you take a look at some of the "Lower Build Kits" at Palmetto you will get an idea of all the parts required. Most of these packages include everything you need to build out the lower:
http://palmettostatearmory.com/ar-15-05 ... -kits.htmlA trigger can range from what's included in the build kit to $250+. What you need depends on what you plan to do with it and what you feel like you require. As I said before I've got a Palmetto EPT trigger that came in my build kit. Out of the box it was 5.75lbs and not enjoyable to shoot. I put on JP Enterprise springs (JPS3.5) for $10 which brought it down to 3.5lbs and now I feel like I've got a pretty good trigger. I'm happy enough with it that I'm doing the exact same thing in my second build.
For the upper a few make "parts kits" but if you are doing a free float handguard they will include a bunch of parts you will not need. For the most part you will need:
* Forward assist (or plug)
* Ejection port cover
* Bolt carrier group
* Charging handle
* Handguard
* Barrel
* Gas block
* Gas tube