My humble recommendation is look for a complete upper assembly that has your rail top since there are so many upper assemblies available with nearly every configuration possible. I like the Rock River lowers since their two stage trigger allows accuracy that I would have declared impossible just 20 years ago, and their uppers snap into the lower receiver with a tight fit. I prefer the forward assist as I shoot 77grain moly coated bullets, but to each their own.

 

My varmint special is the Rock River Predator Pursuit which uses a heavy match stainless barrel chambered in the .223 Wylde Chamber (allows use of .223 commercial or 5.56 NATO without headspace issues). This upper assembly has a free floated handguard, and retails about $600 with the bolt carrier group and charging handle assembly. I use a Millet scope, and can take out coyotes at 600 yards with single shots with this set up.

 

Cheaper possibilities are use a DPMS upper without the bolt carrier group. If you use the bolt carrier from your other upper, it is critical to check the headspace. That is why I like that .223 Wylde Chamber so much. Good luck.

my own thoughts on RR lowers. RR uses  oversize trigger and hammer pins for their "match" guns.. these pins are only about .003" oversize but they are pounded into the lower. Now only a RR trigger or hammer with oversize holes wil fit in that lower....  and since the lower is the registered part, and alumium..... well I just would'nt buy a part that no other makers trigger can be put into.....

 

 but as for the upper question, get a complete upper with a 24" vamint bbl. and be done with it.