Your decision will be influenced by two factors: how warm you sleep and how nice of a pad you have. You will lose heat to the ground because sleeping bags compress under your body weight. A 15F or 0F bag with a dual pad (foam + inflatable) is recommended. The inflatable is for comfort, and the foam acts as insurance (it’s quite indestructible).
Anything below 0F isn’t necessary. Denali class, not Kilimanjaro class, is a -40F bag.
To keep the bag clean, liners are acceptable. For those especially chilly nights, you can always toss on a sweater.
So, just in case, I’d recommend a 0F down bag. If you sleep warmly, 15 degrees Fahrenheit is OK. Because you will not see your duffel from morning to evening, I like to store mine in a dry bag (and I also line my duffel with a sturdy garbage bag).