One IPv4 would be sufficient and easy to set up. Feels like two separate VPS because Mail and Webserver have no conflicts. If you want to run two IPs you’ll need two eth devices. Set up eth0 for web and eth1 for mail.

Mailbox.org

Use your ISP’s or a trustworthy mail provider’s mail out to send outbound mail. Receiving mail on dyndns and no rDns should work without major problems. Ensure to configure SPF, dkim and dmarc accordingly. This can be a real challenge to get right on such a setup.

Zotero supports selfhosted WebDAV storage

Kurose / Ross, Computer Networking