Depending on what your ultimate goal is with any large-frame AR, spending more on the front-end can be of great benefit. The DPMS SASS is one of the best bangs for the buck for an off-the-shelf rifle...even at ~$2k. Similarly, the Armalite AR-10T is another phenomenal value for the money with sub-MOA performance right out of the box for only ~$1600 or so. Your biggest problem is that $1200 just doesn't go very far in terms of getting much more than an operable rifle that you'll be able to piece something better together on the receivers as your time and wallet permit. With that budget...unless you just want/need something to go "bang" right now, you may be better served to get a high quality matched receiver set and building from the ground up.
With that said...as to your LR-308 desires...they are available on GunBroker (but you'll likely be out more than $1200 at least by a slight margin as the market isn't supporting LR-308 rifles of any configuration that you are looking for at that cheap a price nowadays).
Here is one example I found with just 2 min of searching GB for a 24" SS Model for $1220+S&H, transferring FFL fees, etc.:
DPMS RFLR-308 .308 308 Win 24" SS Bull Barrel NEW! : Semi Auto Rifles at GunBroker.com
That's likely as good as she'll get.