@t >I was thinking making sure emails are sent and don't just go straight into spam is probably one of them
Pleromer has a test email command for that, generally it shouldn't be a problem as long as your mailserver is configured with DKIM/DMARC/all the other bullshit, otherwise just add a warning not to use gmail/outlook for regs on about page.
Protip: if you're lazy, you don't need a while separate mailserver for registration confirmations, you could use cock.li/lolcow.email/whatever with SMTP login support.
>do I need an S3 bucket to handle media etc or is there a way I can host it all locally?
No, unless you're aiming for thousands of users. At worst your media would take maybe 10% of storage space compared to DB, at least as long as you're not doing the sjw and making hundreds of image spambots.
>do I have a system running Proxmox or something similar and then have the instance in that?
If you feel like it. Mine lives in a basic LXC container that can only communicate with host and postgres database in the next container.