As for uppers.....you're talking "carbine" and not "rifle" so I'm assuming you're looking for a shorter gun than the 20" or 24" models. I have short and long guns and the mid or full length gas systems are definately smoother. Some say they are more relibale, but I can't really complain about the short guns I have.
If I were to build a tricked out "carbine" I would opt for the 18" gun with a full length gas system. The longer guns shoot faster on the clock for me in 3 gun at any distance outside of "across the living room" range. You get the extra velocity and sight radius with irons as well.
On top, I'd have a good set of flip up irons or detacheable A2 (stand alone - not carry handle style) front and rear. Eventually, these guns cry out for a red dot or ACOG so it's nice to have the option to cowitness. For the best acuracy in irons, consider the match style globe sights. They are also very fast.
I mentioned the Clark Gator. It also can be ordered in 18". Always an excellent choice. With the aluminum float tube you can add rails.
If you must have quad rails and a tight twist the ADCO guns are a good choice.
http://www.adcofirearms.com/sprbuild.cfm?page=build
Notice they use the White Oak barrels.....
Unlimited budget? Only one place to go.....
http://www.jprifles.com/