Most people assemble stripped lowers themselves, and buy uppers. Building an upper is definately do-able. If you want to take a stab at it, you will need
Stripped upper, ejection port door (spring and pins), BCG, Charging handle, barrel (w/ flash hider or whatever you want on the end), correct length gas tube, gas block (w/ roll pin), your choice of rail (w/ barrel nut)
Rough list off the top of my head. Anyone feel free to correct.