If the QP's don't mind, I'll pitch in here (since your question was directed at them).
Fundamentally, there really isn't any difference between lower receivers, so its kind of a misnomer to label a lower receiver a M4 receiver unless it is completely built, and has their 6-position collapsible stock. All of the other parts except buffer tube and buffer are the same as those used in a 20" fixed stock rifle.
If you want to build the rifle up yourself, head over to
AR15.com's Build-it-Yourself Forum for a list of tools, and illustrated step-by-step instructions. Their equipment exchange forum has a ton of dealers that you can get the gear you are looking for. A good source for tools for working on AR15's can be found at
Brownell's, as well as a good selection of parts to complete your rifle with.
If you want to get an upper already built, and want one of the best in the business, get yourself a
MSTN upper receiver. Or you can get one from
Rock River Arms,
Bushmaster,
DPMS, or
Stag Arms among others.
I would suggest trying and building one yourself if you feel so inclined. MSTN offers uppers in 6.8SPC and they generally see .75 moa accuracy with Remington production ammo. The 115gr SMKs will hold moa to 500 yards. Prices for barrel/bolt sets start at $475.