William. I have just done as you wish to do. I larned a few things . I got a stripped lower. They can run fro $80.00 to $ 180.00 + . You are best served with a mil. spe. as most mil spec parts will fit. There are several types . no fence , partial fence , fenced and M-4 to name a few. Fenced or M-4 are good to build on unless you re enact and wish to match a speical era like VN or something. There are speical tool you can buy but they are not so expensive. The blocks for upper and lower are useful. Now the nice way is to get your lower and build it then order a complete upper. You can configure your gun as you wish. Look in Shot gun news , M&A is a good ad to see the different combo's aval. They will send you an upper complete and lower parts kit. You can upgrade parts as you wish . There are other outlets that do the same and there ads are there also. note I have not ordered from them but did use imfo to decide what I wanted. I have the advantage of a gun show in the area where several guys sell uppers and will build your lower while you watch at no extra charge if you buy the parts from them. The guy I use is Tony's Uppers and he is on line . He said M&A was his brother in law , small world. Anyway he put togather a 223 target gun sorta . It is a 20 inch bbl , M-4 lower Flip up sights and I added a scope. The sorta is the 4 position M-4 stock. It needs to be a solid M-2 IMHO to be a better target gun. That was the beauty now the moles and warts - sometimes these guns are rough and need breaking in and in some cases polishing. I know because mine did , sons did not. It is a work in progress that you do the picking of parts and decide when it shoots good enough.
Anything that is M4 is current military parts for the M$ platform. M2 are the M16 type parts . With regard to recivers a M4 has bullet ramps cut in and a flat top . A M2 can be flat top but no ramps. Ramps only help. For target flat top is a must have. Bolts come chrome or not , best in your case to up grade to a target bolt. BBL's chrome lined , not CL or SS . your case SS or not cromed might be best. Hand gard for target would be free floated , I like a full length round alum. with four3 inch rails on the end. Gas blocks can be with a standard M2 fixed sight , folding sight , with a rail or no sight or rail. Mine has a folding sight. Mine also has a standard flas supp. . Also my gas block has a sling stud. Some do not and with a bottom rail you can attach one. Ok mine works now after polishing the bolt , carrier and chamber. It is heavy but I wanted it that way as it will shoot ground hogs and tagets way more than bug out ( I hope) . I am now in the process of getting a 6.8spc upper to work. The gas port in the bbl was to small and the chamber was rough. The upper had to go back for cleaning up. This is where a good supplier is important. You are getting mil. spec. parts and putting them togather some need attention . The first bbl. I had looked like a sewer pipe after cleaning and it was replaced . Note the bbl camr from a well respected bbl maker . The volume of production may allow a bad one to slip thru. and would get inspected on an assy line. When you build you are the inspector. So in the end after all you will have a rifle you put togather with help. You decided what parts went in you decided when it was the way you wanted it. You saved some money , you really do! I saw one like mine in the store for around $1300 and to date mine is under 850. Would I do it again ? You bet ! just need funds and to decide what cal.