I'm going to have to defer to the experts on this one. I would not do anything until you find out for certain what material it is made of. It could be a resin cast or a fiberglass helmet. I readily admit that I can't tell what it's made of.
My general suggestions would be:
Don't cut the T-slot out until you've finished all of your prep-work: sanding, cutting etc. It helps support the helmet. You could paint the T-slot visor area black to get an idea of what it will look like, but don't cut it out until you're ready to install a visor.
I'd recommend a Shade 5 welding shield instead of buying a ready-made T visor.
Get a dark green welding shield if you are making an ESB Fett, Smoke Gray if you are making a ROTJ. If you're making a Pre-Production suit, the color is up for debate, but I bought a dark green one for my PP2 helmet.
You will need to know what kind of material the helmet is made from so you can choose the right kind of filler ie: which type of bondo.
I think with some patience you will have a great looking bucket! :cheers
I'd really love to see progress pix of your helmet. BTW, which version Fett are you planning on making?