Plywood over floorboard method. Also SPC flooring anyone?

Guys,
Finally taken the plunge and removed the kitchen floor tiles and ply fron under it. Now only left with the floorbaord.

Want to plywood it over before flooring can be laid on top.
Any general tips or guidance to follow in how to lay down the ply?
Screws or nails?
Which thickness?
Which size board cuts to cover the floor?
How much space between each board?

Then for the flooring. Anyone have experience with Spc flooring please?

Thanks