pepsimayn, you do have several tough choices...barrel length, profile, weight; to ras/sir or not, and buttstock style, being the most noteworthy. Once you narrow it down as to what you want, the rest is relatively easy.
The 3 kit companies most people cite are:
J&T
Model 1
M&A
The mix 'n match upper/lower thing is good too, that's how I got my first AR...Eagle lower and Armalite upper.
The best way to assure you're getting exactly what you want, and not paying for stuff you don't want, is to build the upper as well. It's a little more expensive that way, 'cos there are a couple of tools you need...action block and armorer's wrench, and the 2 of those will be about $60 or so. The combination of parts might be a tad more expensive as well, but it probably works out in the wash.
If you ever think you'll build another rifle, or just want the ability to swap barrels, then you'll need these tools anyway. By building the upper, you'll be able to put rail style/free floating handguards on before you get the thing assembled. BTW, stay away from muzzle brakes if you do want to do this; it's MUCH easier dealing with this kind of stuff if you don't have to worry about removing a pressed on or soldered on brake.
A lot of your parts you can get from either Midway or Brownell's, both of whom offer discounts to C&R and FFL holders. Brownell's has great prices on the DPMS competition style--no forward assist, and no trap door--uppers...they have one for $80, and another for $88. ADCO has complete RRA uppers, with the forward assist and the trapdoor stuff already installed for $90.
If you really want the M4gery, ADCO has the RRA M4 entry uppers for $411.78 delivered. Build yourself a lower for around $225 (with transfer fee on the lower)or so depending on buttstock, and you're ready to go.
Good luck.