Skyno, there are 2 camps regarding using a stone in the lid. There are those that use them and those that do not.
I think you can get great results with either method. Those that don't use a top stone, will simply attach the ash catcher tray under the lid.
I may be wrong, but I think member Bucierasdon uses a saltillo stone on top with good results. If i remember correctly, Ronzo uses a cordierite stone. Others like Essen1 have also added insulation to the lid above the lid stone.
In my MBE, I have done tests with a stone versus an aluminum plate and didn't find a difference for my particular setup, so I went with the plate b/c it was lighter. I now have an LBE which is set up very similarly to my MBE so I just attached the ash catcher.
Personally if you are interested in using a stone, I would try with the cheaper materials first and if it doesn't work then upgrade. No sense in trialing an expensive $30-$40 stone when a cheaper alternative may work. Because the top stone doesn't get direct heat, I don't think you have to worry about chips flaking off, but it is something to consider. You wouldn't want to bite into a stone chip that is hidden in the cheese or toppings. If you have an extra pizza stone laying around and some ceramic drill bits, it may be perfect for that purpose.
If you look around, you can likely find a 12" cheapie pizza stone for around $10 at Bed bath and more or Sears that should work fine.
Good luck and do post up some more of those good looking pies.