In answer to the question: How much fabric to buy for borders and binding?
This is one of those questions where the answer is "it depends"
I prefer to use strips cut lengthwise for the borders and I prefer not to piece the borders. Of course, if I'm short of fabric, I am perfectly capable of improvising.
Do you want to miter the corners or not? Do you put the side borders on before the end borders or vice-versa?
Does the pattern you are using give any guidelines for how much to buy?
Or are you improvising?
I usually put a fairly wide binding on my quilts - and I cut the strips about 2.875 inches wide (2 7/8") I like to cut my binding fabric on the bias. Do you cut your bindings on the bias or on grain?