Will you be using W.O.F's helmet templates? If so you can build it out of heavy grade cardboard. Antman has a great build tutorial. Or you can build it out of signboard (Sintra) but I'm not sure you can get that in Poland. If you build it from Sintra then you can leave it as is, or you can make a mold from it and cast it in PVC resin or Fiberglass.
Do a search on helmet builds and you will see the different methods members here have used to build their helmets. As for the visor you can order one from T visor.com or Bobamaker.co.uk. Your other choice is to get a green face shield used for grinding and brazing (grade 5) and use that as a lot of people here have done.
Your choice of 2mm PVC sheet should work providing it's flexible enough and can be heated to shape. Don't forget to wear a respirator when doing it as that stuff throws off some nasty vapours.
Hope this helps