Without doubt, mind that statement above about the pad. It is that important.
I find that there is such a thig as TOO warm a bag. I don't believe I have ever actually zipped up my bag ( a great down bag from REI ). It is too warm. It is rated to -10 or -20 and I believe it. If I were to do it over, I would chose another, slightly less capable bag.
As for whether it should be down or poly fill, or canvass covered. . . all of that depends on how you intend to use it. If you are camping within sight of the trail head, heavy doesn't matter. If you are camping in a tent or camper, on cots canvas or outer covers do not matter. Get the picture ? Match what your needs will be.
I might suggest, make a liner. Take a flat sheet, fold it in half lengthwise and sew up the bottom and about 1/3 to 1/2 way up the open side. Place that inside the bag first and sleep inside that. That liner is MUCH easier to clean than the bag is.
ChuckC