Zero. My first grow was in a water softener box with aluminum foil on the inside. Light was LED strips hung with mechanics wire.
Yes. PVC inner frame primarily.
Plywood is expensive. Figure out how big a box you need and figure out how many sheets you need. Simple. Chances are a tent will be better. Or panda film a corner in your basement.
I use old school math, but you can use new math if you like.
Nope. TBH on that tight a budget I would start saving and be patient. Wait until you can setup something legit instead of wasting your money of something you won't be happy with and will end up replacing. This goes for the whole grow, especially lighting. But thats just me with 30 years of growing under my belt.
