Hey Jeff. This caliber is pretty amazing to shoot with its low recoil and inherent accuracy. I've heard good things about Grendel Hunter on other forums, but don't have any experience with them. Lots of great info on this site, so it may be worth doing some research here if you have not already. 300 yards won't be a problem with a Grendel in almost any barrel length - with quality components and decent optics. Most folks here will probably tell you there isn't a real advantage for for 20" over 18". I have an 18" and it shoots great. I've taken deer with it at just over 400 and was able to ring steel out to 760.
There are a ton of options out there. If I was looking at a complete upper and this price range, I'd look hard at Larue's kit and have them assemble it.
There are a ton of options out there. If I was looking at a complete upper and this price range, I'd look hard at Larue's kit and have them assemble it.