sort of depends what is left of the stain, and how the wood looks. pressure washing tends to raise the grain in the wood. 80 would probably be a good place to start, belt sanders are pretty aggressive. Make sure all of the nails or screws on the deck are recessed before firing up Mr Scratchy, or you might cut the belt.
After an aggresive grit, something like 120 or even a little higher will make the wood real nice and smooth. Should look brand new if you are putting this kind of work into it.