Is it necessary for the acetone used at this stage to be anhydrous too, or will normal 99% acetone do?

Yes for everything. The only use of acetone that need not necessarily be dry is when evaporating. Generally, it 's advisable to keep SWIY's acetone as dry as possible, especially before use.

SWIM's going to be working on a way to dry acetone with anhydrous magnesium sulfate in THP. It would be the same idea as running the acetone through a column of desiccant. He just needs a sufficient container to pour into.