M16 build.

[Yournamehere]

I'm looking to make a semi auto M16/AR-15 build, preferably a retro A1 style build (since the actual M16A1s cost over 10 grand they are out of the question). Does anyone know of the ease of acquiring an A1 upper or lower, stripped or complete? Also, if your upper is safe for 5.56 (like an A1 upper) and your lower is .223 only (say an SP1 lower), is it safe to fire 5.56x45 ammo out of the gun? Any comments, advice or suggestions on M16s are welcome as well. Thanks.