1. any stable linux or BSD should work, ideally choose one with a big community so you can search support and guides easily
3. keep in mind you don't need to buy a pi for this, you can use an existing PC
5. wireguard is good for this, you can use this easy setup too 
GitHub
GitHub - wg-easy/wg-easy: The easiest way to run WireGuard VPN + Web-based Admin UI.
The easiest way to run WireGuard VPN + Web-based Admin UI. - wg-easy/wg-easy