Ah, the final element. The cherry on top, if you will. I'm all for different preferences, but unless you have an allergy to down, the down comforter is the holy grail of mattress coverings. The right down comforter will do the most to envelope you in a world of snug, and you can use them year-round depending on the weight of the comforter you buy (look for an all-season comforter if you want to use it year-round).
While you don't have to blow your budget on every bed element, a duvet cover is a worthy splurge. If you don't go for a quality down cover, you may end up with one that has shifting feathers (leaving you cold in spots at night), or feathers that poke through (not a feeling that keeps you dreaming). Duvet covers are optional, but I believe they make your bed feel even more luxurious, and it's a lot easier to clean them than an entire comforter.
Still not convinced to go the down route? The Buffy comforter ($160 for a full/queen) is made with eucalyptus fibres instead and keeps you nice and cool at night. Either way, you're about to embark on your most epic journey of coziness. Sweet dreams.