The soil amendments you mention all sound good. How much you add depends on the condition of your current soil. If your soil is mainly heavy clay, you need to add amendments that help loosen it, such as coarse sand, peat moss and, of course, compost. Generally, I try to incorporate three or four inches of appropriate amendments into any new beds I dig.
Fresh manure burns plant roots.
And if you're adding peat moss, moisten it first. And incorporate all amendments well before planting. Willows like moist soil, and most do well in partial shade. Weeping willows grow into massive trees with wide-ranging root systems. If you have a large property near a natural pond or stream, go for it.