Will it be hand quilted or machine quilted? If machine quilted, will it be on a domestic machine or a longarm?
I would back the quilt with cotton. Silk backing would be too slippery.
I have also heard of silk tops being created with a flannel backing, as the flannel will not slip and slide around. Flannels are wildly variable in terms of pilling over time, so I would definitely prewash at least samples of flannel to make sure the quality is there before using as a backing. If quilting will be moderate to heavy, then it is not necessary to preshrink the flannel as the batting will determine shrinkage. However, I would probably preshrink the flannel anyway (washing and drying twice).
Silk batting is wonderful for both hand and machine quilting, but pricey. It is soft and lightweight too. You don't have to use silk batting though. A thin 100% cotton batting is good because it will continue to get softer over time. Quilter's Dream request or select would be good, as would Mountain Mist Blue Ribbon 100% cotton. I would stay away from battings with scrim, such as Warm and Natural, because of the added stiffness. With a silk quilt top I think you want to stay with something thin and soft for batting, and a backing fabric that will not be slippery or too heavy.