Structural parts of plane, balsa, carefully selected, is all you NEED to build a VERY good airplane. The trick is in 'carefully selected'.What are the best or necessary materials?
Selective use of small amounts of carbon fiber can reduce how carefully you need to select your balsa. It can make the plane a little tougher, particularly carbon fiber wing spars. But frankly, this is NOT the part of the plane I generally break.
It can be useful in stiffening the motor stick, but there are other ways to get there with balsa (hollow motor sticks).
Wire is about the only choice for your propeller shaft and motor hook. But you don't need massive diameter. I typically use 0.020 to 0.025 diameter music wire.
Propellers, balsa allows light blades, is fragile. Sheet plastic will work for these planes for the blades, read back up this string to see discussion thereon.
Covering. I have used dry cleaner bags, produce bags, grocery bags. The trick is to get a collection of these from local stores, weigh and select the material that is lightest per square cm, inch, foot, whatever. You do NOT need specialized indoor mylar films for these planes. It's not hard to find a material that costs less than 0.5 gm of your 8.0 gm budget for this years rules. I used dry cleaner bags this year and it still allowed me to use what I consider incredibly dense/strong balsa (7-10 lb/ft3) for my structural parts. And my plane came in at 7.5 gm before ballast.
I've updated my Bill of Materials for this years plane if someone can show me how to load the excel file or a snapshot to the site.