Others here have WAY more experience on scratch-building than I do,

but here's my two cents worth:
Depending on what thickness Sintra I was cutting, I used different tools. I'm not crazy about my no-name dremel knock-off rotary tool for cutting. 3mm sintra I cut with either a jigsaw or a circular saw, fine tooth blades for both. I also use hand tools like a coping saw for sintra and a hacksaw to cut my batting helmet in half.
When I was cutting the For Sale signs, I used a sharp pair of scissors for the curves and an X-acto knife for the straight cuts. I just lay a straight edge on the line, lightly cut it and then score it a few times. It snaps clean after that.
In my painful experience, I'd suggest For Sale signs for the helmet and 3mm sintra for the body armor. The For Sale signs are more forgiving and easier to wrap into the shape of the helmet than the thicker sintra.
I'd suggest having someone around when working on this stuff, too. An extra pair of hands is always good to have around.
BTW, welcome to the Fett-head asylum! :cheers