JD has pretty much hit it right on the head...what will you be shooting out of it? Handloads, factory plinking ammo, match ammo, etc, - also, what grain?
How heavy or light do you want to keep this?
Those factors will have more impact on what you are trying to do, versus what options are hanging off of it, etc.
Barrel length wise, you probably don't want to go shorter than 20", and keep your twist to 1:8. That will allow you to go to the heavier rounds, say up to 77gr. and down to 50gr. with little or no problems. Any heavier and you'll be looking at something in the 1:7 twist range, lighter you'll be looking at dropping down to the 1:9 or slower. Then you might have a problem keeping it all RRA.
They have the Varmint EOP, the Varmint A4, the Coyote and the Predator Pursuit uppers.
- pretty much sums up the options you'll have with RRA. Again, it pretty much comes down to finding one that will meet what you want it to be able to do performance wise and ergonomic wise. If you are going to be moving around while you are out "varminting", then consider the extra weight that the bull barrel in 20 - 24" is going to add to your set-up. What about a bipod? Again, added weight. And any optics you add - again weight. This sucker gets heavier by the second, doesn't it? And we haven't even added magazines full of ammo yet.
One of my "varmint" pieces is set up just that way...a 20" bull barrel, 1:8 twist, free float carbon graphite tube, 2 stage match trigger, with Harris bipod, and 4-16x optics, and a Magpul PRS stock (althought that might be coming back off in favor of an A2 stock - for weight savings). It will pick ticks off a coyote's sphincter at 1-600 yards all day, assuming you do your job with trigger control, breathing control, correct ammo, windage and elevation, etc. But...it's also a workout if you have to lug it around the country side, as it weighs close to 18 pounds fully loaded.
Not that 18 pounds is un-doable or bearable. It just gets old REAL fast. But if you are going to be stationary, then it's not a problem.
I also one set up with a 24" bull barrel, again with all the bells and whistles on it to be able to reach out and touch something - a long way out, and do it consistently, or at least until the barrel is too hot to touch or I run out of ammo. The 24" set-up I have is even more fun lugging around. I've often thought about converting one of the old golf bag carts in the garage just for that purpose
Again, it all comes down to what your requirements are. Another option if you can break free of the "keeping it all RRA" is going to .204. Lighter set-up, just as flat (maybe flatter trajectory than some .223 ammo). But might have to wait for RRA to jump on that band wagon.
As for waiting for an upper...not necessarily the case. Check out this link...
If it's shown on the site, it is IN STOCK. And if it's not there, wait a little bit, because they have been getting stock in regularly. I've done biz with them for that last couple years with fantastic results and service, usually getting my order in 3-5 days, and would highly recommend them.