Isnt't port 81 where usually the nginx proxy manager webui is served? I think you should just forward the requests directly to port 80 and 443 respectively.
Will check it out. Setting up postfix + dovecot with dmarc and postgres was a funny experience but it's starting to slip out of my memory how I did it and I don't want to be through it again.
Try the pi for tinkering since it will be cheaper. If you end seeing issues with performance for the usage you need you could start looking up used laptops or optiplexes.
I had some used componentes lying around so I frankesteined a server with used parts after buying some disks
The interface looks nice but I'm not too keen on the network and location permissions. I really liked Simple Gallery for just a "show my folders" view.
Bully behaviour