Ben,
Where and when are you planning on taking your first trip?
That's a pretty good way to choose a bag.
There should be no problem for you both using a down bag in a Hilleberg. You would need to work pretty hard to get it wet enough to be a problem in there, and the weight and comfort range of a synthetic just won't compare.
If you're planning on mostly camping with your wife, you might consider getting two light weight bags for summer, and a light quilt to go over the both of you in your summer bags, for when it gets colder.
The other plan is to use a down filled mat to help boost the temp rating of your bags.
I'm within a couple of centimetres and a couple of kilos of you and this is what my wife and I use.
Summer trip
Me Western Mountaineering Summerlite plus Big Agnes insulated mat
Wife who sleeps cold Western Mountaineering Apache plus Big Agnes insulated mat
Often I unzip the Summerlite and use it as a quilt/sheet
Winter trip no snow -5C
Me Western Mountaineering Summerlite plus Exped Down 7 insulated mat
Wife stays home under doona
Winter trip snow -5C to -15C
Me Western Mountaineering Apache plus Exped Down 7 insulated mat
Wife stays home under doona with electric blanket on
I reckon if you went with a WM Summerlite, Megalite or Ultralite, you should be fine. If your wife is a cold sleeper, get her the next rating down to yours. If you're over thirty and haven't done much walking, I would definitely get a pair of blow up matresses (Big Agnes, Downmat, POE Ether or Neo) rather than self inflaters (Thermarest etc) The comfort really can't be compared. If you're still under thirty, you can probably still tough it out on a Thermarest.
Paddy Pallin's in the city or Glenferrie Rd stock most of these and Bogong and/or the Wilderness Shop in Box Hill have the rest. You'll need to order on-line from the US though, if you want to keep it under US $1000.
Stick up another thread when you're ready to order and we might be able to make suggestions of on-line retailers we've had success with.
Rod