Something you may like that one doesn't usually see at their local retailers are ready made sleeping bag systems. They consist of a lightweight bag, medium weigh bag sized to fit over it, and a Gore-Tex bivy (bivouac) cover/bag sized to fit over both. They can be used in various combinations for warm-dry, warm-wet, cold-dry, and cold wet weather. You only have to carry what you need even if it's just the bivy cover to stay dry.
In the USA, we can buy the actual
US G.I. side zip sleep system issued to our conventional troops on the commercial market.
Due to my background in Spec. Ops., I'm partial to center zip (opposed to a side zipper) bags with quick releases. If I don't engage a securing tab, simply pushing my arms out against the insides of the bags forces the zippers to slide open nearly instantaneously enabling me to present a weapon; or, get out quickly even when I'm not completely awake. I own a
SNUGPAK system to which I added a Brit. G.I. Gore-Tex bivy cover.
If you don't anticipate being in a hurry to get out of your sleeping bag, a side zip system is just fine and has the advantage of not needing a baffle along the zipper to prevent heat loss as body heat rises to the inside top of the bag.
Sleeping bag systems like these make everything else just a matter of how much you're willing to carry for additional comfort. If you're inclined to go with wolf's option, I recommend a
Black Ops. Survival Kit.