Bags are rated in F.

If is is rated to 0 it does not matter if it is inside a tent or not.

A 0 rating means you will start to feel cold at 0. If you want to be warm at 0 then buy a -5 bag.

Get a down bag that has a shell that is highly water resistant. Mountain Hardware has bags with their Conduit SL. This they also use in jackets / shells.

The rating should apply with a base layer on.

Powerfill is a rating of down quality; the higher the number the better quality and therefore the down will be loftier. A 800 fiull bag will be lighter and more compressable than a 600 fill bag.
800 will cost more.

If you want to be toasty get a 0 bag. Personally I would use a 15 bag and augment it with more clothes.

If you want to make a bag warmer than it is rated just wear more clothes. The sleeping bag industry had this myth going that you needed to be undressed. This myth just allowed them to sell heavier bags which equals more $.