If you buy that upper, you better have a registered SBR or pistol lower to put it on or you will be in violation of Federal law. The upper has to have a fixed barrel length of 16" to be legal. If you want a removable flash hider, have a 16" inch barrel. If you want a shorter barrel, the FH better be fixed.
Nothing about that upper sounds "Rare" to me either. If you want to build it yourself, buy good tools and get a quality kit and milspec lower. If not, buy a good carbine; remember, you get what you pay for.
CMMG was selling some M4 clones of decent quality on their website for $550 a few days ago; I would consider one of those for a beginner. I have been thinking about getting some just to stock up on.
My 2 cents.
When in doubt ..... mumble!