Get the best down filled bag you can afford. There are several categories of down filling. Generally, <500 fill is about the same as a good synthetic. 500-650 is medium quality, 650 to 800 is good quality. (I am using EN ratings, maximum is 800, US ratings go up to 900.) Eider down is best, very expensive, around $3000. Wild Canada goose is second best, difficult to get as is many duck species. Graylag or domestic goose is third best, but most down comes from these birds as a result of meat processing. Eider down is usually rated at 800+ on the EN scale because it traps air better than greylag down and is more water resistant.
Temp ratings are not absolute. Get one that will keep you warm. Some will do fine with a WM summerlite. Some will get cold under the same temperture. And this will vary between male/female and ages. Personnaly, I use a 40F bag down to about 25F alone. I wear a down jacket that does double duty as camp wear, also.
A good bag will also have a good down/water resistant/breathable shell and a soft down resistant/breathable lining. Maintence is the key. Wash them every 14 days of use to maintain loft, or warmth (loosly speaking.) Special down wash, never a detergent (hence, never buy one used) or in a commercial machine that has any residue. Run a rinse cycle first. Never handle the bag wet, the baffles are very fragile. Dry carfully, so as not to damage the liners (inner and outer.) Use tennis balls, not sneakers...better is to use the purchased dryer balls. They take work to maintain. Drying in the air is recommended for a couple days before storage. Ideally thay should be hung and covered with a sheet, but put in a larger cotton bag is good.
Suggest a week or so of study before any purchase. Marmot makes good bags, lots of features, but a bit heavier. Do you need this? A cuben quilt is simplistic and adds a vapor barrier...do you always sleep at 30F and below? Several gimmicks out there too. Waterproof down, difference in baffles (sown through, angled or square, continuous or not) difference in fabric covering and inner linings. If you don't know what I am talking about, are you ready to drop $500-600 on a high quality bag? You don't need to be able to make one, but you should understand the differences, and why there ARE differences.
Then you will be ready to buy.