I would say don’t go right up the the lower limit of 2GB, unless you are running a small forum with not many users.
No, cPanel is not needed.
Does the business email provide a transactional mail service? I know some don’t. It’s safer to use things like Mailgun, Brevo, Amazon SES, etc.
Others have elaborated on this, but I’m linking the documentation for this: Configure your hosted Discourse site to run at the root or apex domain (e.g. example.com)
That said, I would suggest you just go with the normal, default option of using a subdomain. Plus, if you already have your main site under the root domain, it makes it obvious that the forum is under that org and not a random one on the net.
A snapshot is essentially a complete backup of the whole VPS. You can still take backups of Discourse regularly in case something goes wrong, if you’re only hosting Discourse on the VPS.