An AR-15 could only be converted to select-fire by someone who was both a master engineer and master machinist.
I'd give a second year apprentice four hours to machine an already finished semi-auto receiver to accept the auto-sear. (and that's factoring in a lot of screw-off time)
I believe the trigger, disconnector, auto-sear, etc, can be bought in a kit, or made by the same apprentice without too much difficulty.
Of course all that is illegal as hell without the proper paper work, so don't do it. My point is that is doesn't take a master anything to convert one; just a little skill and a couple drawings.