What is the material requirement for Van der Graff generator sphere?

You want to deposit charge inside the high voltage terminal.

You'll need some mechanical strength for that and because the charge is deposited near the middle you'll need an conducting path to the surface.

It wants to be very smooth and every radii as big as possible.

I think the 4 MV one I worked on had a dome of about 1/16" stainless polished to a mirror finish. After 34 years it had quite a few dings.

Here's a photo of the dome. The rings to the left go around the structure to control the potential gradient to the base. The top deck is supported by layers of glass and metal (square columns) and the beamline in contained in the round glass and metal structure. The long acrylic rods passed from the base to the deck to control variacs at the high voltage terminal. The terminal could either go to +4 MV or -4 MV.