Man-made fibre bags which are warm enough for what you want will not pack down to nothing. They also tend to be on the heavy side, approaching two kilos isn't unusual, although you didn't mention weight. Good down bags are better from those points of view but they're expensive and you don't want to let them get wet. Make sure you aren't allergic to down before you spend the night in a down bag.
A night in a sleeping bag that's too warm for the conditions is more uncomfortable for me than one that's too cool because I get drenched in sweat. You can layer sleeping bags like you can layer clothing, so I have more than one sleeping bag to choose from and they weren't all expensive.
Beware the temperature ratings that are bandied about by people who sell sleeping bags. The minimum temperatures they quote are usually for survival not for comfort, you can add at least ten degrees Celsius to most of them for any kind of comfort, and more than that depends on the individual anyway.
You compress parts of the insulation of a sleeping bag when you lie on it and that significantly reduces its effectiveness, so what you lie on is easily as important as what you sleep in. Look into sleep mats (possibly under-quilts for a hammock, but a mat will do the job and will work on the ground) as carefully as you look into sleeping bags. Mats can collect condensation so consider that the base of the bag might need to be something like Goretex if you have a closed cell foam mat or the bottom of the bag might get damp. I use a bivvy bag to avoid the problem.
There's a whole religion about what, if anything, you wear when you're in a sleeping bag. I'm firmly in the clothing, er, camp.
