Double star makes a cheap lower and upper (stripped) that's pretty good quality. I got them for $100 a piece, and their customer service is excellent. Be a good place to start.
As for stocks, magpul obviously makes good ones, but you have to factor in a buffer tube set up as well. I ended up going for a Rock River Delta cause it had the same feel as a magpul stock and it included a buffer tube set.
For parts kit, again double star is cheap and good quality. Besides, I've been told by a couple people that most basic parts kits are the same in terms of basic quality and function.
BCG and charging handle I'd look into Bravo Company Machinery.
Barrel and handguard... Depends on the application. If you want to spend some big bucks White Oak Armament and Noveske make some Match grade barrels (and matched BCG's).
I went with a Rock River 1/9 carbine barrel, just cause I got a huge discount when I bought it, plus it came with a front sight tower already pinned.
Handguards, well there's about a million options. YHM makes a good free float for around 100-150$. But just about every gun company on earth makes a handguard set up. That's a good place to start.
Unfortunately, I think your going to be hard pressed to find anything in stock at this point. When everyone went gun crazy most everything went on back order for a couple months.