I would get the gasket set from VW. For a little more money than Halfords or GSF, you then know you have the right one at a good quality.
As for knowing how much has been taken off your head, I would take the head into VW and ask them to measure it for you. The chances are it has never been skimmed, but you will not need much removed to make it flat. Its purely a re-surface. But then again, you don't know if a previous owner has had it skimmed for extra compression.