Official doc is here: https://docs.mailcow.email/post_installation/reverse-proxy/r_p-nginx/ (ngnix is a more common choice for reverse Proxy). would recommend using only one service per vps (unless you're really on budget), activate Backup/snapshots.

If you have the knowledge its fine, if not it wont be fun. mailcow ist the best overall package i have seen.

Proxmox is Debian.

Finally we are on the level of Linked.in, do you need Karma?