1. Yes.
2. Doesn't matter
3. Setting up Phoenix/TeamRed/LoL takes ~5 min if you can read basic instructions and copy and paste your wallet address.
4. Good luck finding any. Probably the 3060ti,3070,or 3080.

Don't blow up your risers by powering them with sata, don't skimp on power supplies, don't invest more then you're willing to lose, etc.