I would think you'd be fine, but I am not certain what night time lows you are starting out in. The real criteria you should be using is the range of conditions you can expect where you are going, and since I'm not familiar with that area I'm no help there.

The Big Agnes insulated air core is probably good to about freezing, lower if you add a CCF to it. The bag I am unfamiliar with. However, consider that at colder temps, if you don't turn out to be a warmblooded individual (all these things are relative, comfort levels vary), having the sort of bag where the pad is inserted into the sleeve will mean it's not going to be possible to wrap up in the bag as it would with the more standard bag-on-top, unattached pad combo. If the bag is not close fitting to your body, you'll be rolling around in an air space which your body heat will have to keep warm, so the colder you go, the more work your body has to do at night to keep that air warm. This probably won't matter much until you are approaching the advertised limit of the bag, which may or may not be accurate for you. 600 fp down explains the weight of the bag... perhaps not the best quality down out there, but should be warm enough for your purposes.

The gear will work, no doubt, but is a bit heavier than it needs to be, as you say. If you got a good bargain on it, use it for a while - if you at some point want to cut the weight and have the money, and really enjoy the backpacking to the point that you can justify the cost (or not, some people need no justification) a Lightheart 2 person tent (or Zpacks, or Tarptent) will cut the weight of that item. Higher quality down bags like Western Mountaineering or the 800+ fp Marmot bags can trim some weight off the sleeping bag. And the Gregory, once you've reduced the bulk and weight of the rest of the gear, could easily be replaced by some of the lighter weight packs - not frameless, but not built like a Mac truck either.

But that's all money you don't have to spend yet - and the gear you have would likely sell for a good price used, if you don't beat it up a bunch. I often recommend rental gear to people and that's not light either. It gets you out on a short easy overnight to do the important work of testing yourself, seeing how comfortable you are with backpacking, and having a more bombproof tent while you are doing this is not a bad thing; some of the lighter shelters take some skill in some environments to set up in weather.