Your first intuition, will probably work for you. MB makes great jackets. For sizing their charts are accurately snug...ie if you're in between sizes or want a little wiggle room around your chest, size up.
Look at the rab infinity jackets in this fill weight range too, but I can't attest to their fit.
At ~7oz of high fp down you'll probably be warm enough down to the single digits with some minor camp chores. You can actually probably get by with just 5-6 oz of down though, especially if you run hot. At this weight range you get a lot more parkas/hooded jackets (make sure you're getting a hood, just to be explicit) available for less than $300. The brooks range alpini anorak can be found on a few sites now under $200 and the GoLite Bitterroot is only $240 for 5.3oz of 850fp (bitterroot is basically a MB Mirage, but $100 cheaper).
The SD is only 650 fp and you'll definitely notice the warmth difference between 650 and 750+ down. I really wouldn't bother with any of the big companies that don't quote fill weight. They're selling perCEPTION not perFORMANCE in my opinion.
Now if you want to get really creative and have a more versatile kit for your Georgia use, I have some a suggestion. Currently you can snag a Stoic Hadron from BC.com for 50% off. I know this breaks my rule about unquoted fill weight, but it's widely accepted as being around the same warmth as the MB Down Inner. This will set you back $90, leaving $210 in your budget. That happens to be the exact same price as the Montbell Alpine Light...widely touted as the gold standard for cold but not arctic temperatures. So for $300 you got two pieces of insulation weight in at around 23oz with likely 6oz or more of down (the vest version supposedly has 1.7oz of down). You could also swap the MB Alpine for the GoLite Bitterroot for a bit over budget but some more warmth in a lighter package....
As for weather protection, if you're in the low teens you'll likely not see rain so most any jacket with down proof nylon shell and DWR will shed the Rockies dry snow easily. Shouldn't be too worried about that. Of course if you are worried you could replace the inner in the two piece system above (Hadron) with a synthetic piece for greater weight but better moisture protection.