I totally get the authenticity thing, I am exactly the same way and spend tons of money on my costumes to make them as legit as possible. As for the chainsaw, I say you have the right idea with renting one. Nothing will even come close to the real thing. Now the arm guards-that's what they look like to me, I found a pic, let me know if this is what you mean. If so, maybe there is something in sporting goods (shin guards? as a prototype.) You can make them out of leather, or better yet naugahyde.
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View attachment 7168