Please bear in mind these are just my thoughts and not meant to offend. Unless you're simply planning on sewing squares together, as with a 5 inch charm pack to make a quilt ((which can be very pretty)) you have to plan on cutting, especially with layer cakes - - - and adding additional fabric. I think of it this way: when I buy a box of cake mix, I know I have to add other ingredients to make the cake. The same is true with pre-cuts, especially layer cakes. ((no pun intended)). I think layer cakes are a great jumping-off place because it gives you one or two pieces of an entire collection. Besides, it's a brilliant marketing strategy on the part of our beloved fabric manufacturers. (Thanks Moda, RJR, Robert Kaufman, etc.)
As for patterns, others have mentioned Moda Bake Shop and MSQC is a great resource for patterns. Again, great marketing on their part.
Bottom line, you have to plan on doing some cutting and adding fabric. The pre-cuts are the 'box mix'.