I would recommend buying a complete lower receiver from LMT or RRA, and start from there. LMT is about as good as you can get for upper and lower receivers. A complete LMT lower with the standard M4 collapsible stock should run about $330.00. You can then build your upper receiver to the exact specifications that you want.
If you can be more specific on your application for the rifle you want to build, I can give more advice on exactly what configurations will suit your needs best. A standard chrome-lined 16" barrel will shoot MOA with good ammo, so unless you are looking for ultra precision sub MOA accuracy, I recommend a 16" carbine or midlength gas system chrome lined barrel. There are several options out there to consider.
Building your first AR from a "kit" is do-able, but can be an adventure if you dont have the proper tools for the job(s). There is also an issue of liability that unfortunately could arise if you had to use the homebuilt rifle to defend you or your family. Sad, but true, and unfortunately something to consider.
Buying a complete lower and/or upper from LMT gives you the advantage of a solid warranty with a good company with great customer service to fall back on in case of any issues. If you build up a stripped lower from a kit, you will be on your own unless you can prove that the issue is due to a faulty lower receiver, and will most likely leave you on your own to fix the problem. That can be a pain in the ass with little or no experience to guide you. So, for your first "build" I would suggest starting off with a complete lower receiver, and LMT or RRA complete lowers are readily available, and are as good as you can get as far as quality goes. LMT uses the proper "Milspec" buffer tube assemblies, which will give you more options when looking to replace the standard M4 stock to one of the more desireable after market stocks such as MagPul, Vltor, or the LMT SOPMOD stock. RRA, Bushmaster and most other civilian market manufacturers use a different spec tube which will not work with the aftermarket stocks listed. MagPul is now making their excellent CTR stock in a version that will fit the civilian spec'ed buffer tubes, so I would also recommend getting a RRA complete lower receiver with their 2 stage trigger upgrade. Going the RRA route on the complete lower will likely save you some cash over the LMT lower, especially if you go with LMT's 2 stage trigger option. I second G56's recommendation of Eagle Firearms in Wheatridge, CO. They have complete RRA lower receivers with the 2-stage NM trigger in-stock for $350.00 or without any stock/buffer tube assembly for $269.00. If you go with the complete lower with NO stock assembly, you can get a Milspec diameter buffer tube/assembly, such as a Vltor, LMT, CMT, or Colt and buy which stock you want right off the bat, instead of swapping out the standard M4 stock later. This will end up saving you a few dollars in the end. Im sure Chris at Eagle firearms has Milspec buffer tubes that he can mount on a RRA lower for you along with the stock of your choice.
As far as upper receivers go, I find it hard to recommend RRA's due to the fact that they have what they refer to as "enhanced" feed-ramps on their uppers, which amount to them taking a dremel tool to a standard A3 flattop upper receiver and standard rifle barrel extension and grinding extended feedramps into both of them. This likely wont cause any function issues initially, but may cause other issues down the road depending on your application of the rifle. If your going to get a complete upper receiver, I suggest going with one that EITHER uses a standard rifle barrel extension and a standard A3 upper receiver, or one that uses a real M4 barrel extension, and a real M4 upper receiver. I have seen some RRA uppers with their "enhanced'" feedramps that looked well done, and I have seen others that look like they were done by a chimp.
As far as recommending a standard A3 (flattop) upper receiver goes, Stag Arms(CMT), or Bushmaster would be good choices. I recommend LMT, or Bravo Company for M4 uppers with true M4 receivers/barrel extensions. Sabre Defense seems to be quality as well, but I have not had personal experience with their products. They offer a few different configurations with their M4 uppers, such as Mid-length gas systems, different barrels profiles and different twist rates (1/8, 1/7, or 1/9)etc. If you have any more specific questions, feel free to ask :wink:
Here's my duty carbine:
And my 6.8 precision rifle that is currently in pieces awaiting my new LaRue "Stealth" M4 upper receiver: