Congrats on your demonstration of asking folks here at AR15.com. You will save lots of money and aggravation. Let me give you my take on this deal.
First of all, the other responses are right on target. The cost of a preban lower is between $800-$1000;however, you can get a preban lower mfg after the one you were looking at. That one was a green label meaning it has a large hole upper. Mil spec is smaller. You can find a preban blue label (like mine) that has the small hole lower with raised mag fence for which you can install a detent spring allowing you to throw away the screw pivot pin and go with true mil spec type. That said, lets proceed to the M4 upper. You have several choices here. One, you could go with a Colt, which will be stamped M4 on upper receiver. Or bushmaster is fine too. However, Colt does have current military contract. You can go A2 style where the handle and sights are part of the upper receiver or flat top version where you can obtain a detachable handle if you desire. Once you decide on type of upper receiver, then you have to decide whether or not to go 16" or 14.5" You can put any type of flash suppressor on 16";however, if you go with a 14.5" you must decide on either a Vortex flash suppressor, Phantom type flash suppressor or Kurt Wala's A2 style. Anyone of these permanent attached will be fine for BATF purposes. Kurt Wala is gunsmith under industry forum at the bottom. You must have the flash suppressors put on a specific way to stay out of jail. The uppers above can be had for between $500-$800. The respondent was right about that flip down front site.
You can buy these for around $100-$150 depending on brand. Some need to be factory installed such as GG&G. Bushmaster carrier them too.
The HBAR upper with bolt carrier and charging handle can be found for $400 on this board. I would guess the value of that stuff to be around $2200 at best.
Good luck Erich and welcome to AR15.com