Being your first build, I think you're going overboard from a budget perspective. You've picked a lot of premium parts and from my personal experience, their costs don't add up when you consider the variability within you (yes, you) to shoot within 1 MOA consistently. You oughtta ask yourself, "how good is good enough" then plan around that.
Sorry man... just a reality check to think about. :)
BTW, low mass carrier on a 308? Uh it's not worth the weight savings to deal with the added recoil, especially if you're going with a carbine-length extension with the heavy UBR. Not sure where you really want to go with the build. So many contradicting requirements (e.g. heavy barrel, and lightweight parts).