1) The more the better but I think 2 will suffice.
2) HELL no. You need a portable A/C (discounthydro.com). Plus you're going to need CO2 in your room and if it's sealed you will not be getting CO2. So you're going to have to invest in a CO2 generator too. Fans don't reduce the temps and neither do charcoal filters (still needed in a grow though).
3) I'd suggest 22 dual output pumps with 2 airstones in each bucket (20 plants + a reservoir which needs to be aerated too)
4) 20 plants being on their own bubblers with no reservoir would be a very high matinence thing.
5) Depending on where you live I would be careful of the energy and heat output you produce so that you don't get caught.

Goodluck buddy and keep us posted. Also, welcome to the forums. <3