Hi TeresaP - some thoughts:
Re fish pond - since pond will be connected to a natural water-way, I seem to think you might need a permit (from Environment Agency or local Planning Authority?) to stock with fish. This is not a subject I know much about though, but best you check on permitted stocking before introducing any fish.
Re the marshy bit you mentioned: that seems like the better place to form the pond by diverting stream water to that area, but then I seem to think there might be restrictions on diverting waters from a natural water-way. Again, need to check- perhaps with EA in first instance.
Re willows: some varieties will do better in very wet boggy areas than others - I would suggest you speak to any potential supplier before purchase.
Good luck.