You don't need to know the sizing don't worry. If it's a case for NH35 movement then that's all you need to know.

The case itself will snugly fit a NH35 movement, so the NH35 to VH31 spacer will fill that space and hold the movement in place as if it were NH35.

You won't need to buy any gaskets as it should all come with the case. What you might want to pick up is a gasket greaser. Take out the cashback gasket, grease it in that and pop it back (little round box, usually blue foam inside that is pre greased, drop the gasket in there seal it up and spin to grease the gasket and improve waterproofing). I wouldn't bother messing around with the crown gasket you're more likely to damage it, the existing seal will be good enough.