Put anything you don't trust into their own VLAN. For me that includes the kids pc and iot devices. Give them only internet access with no access to other internal networks.

I have an Exchange vm setup the last 3-4 years now. I use smtp2go for outgoing email. Works really well. I primarily use the system for alerts emails and mailing lists I use. If it breaks its an annoyance but nothing critical. I have a mail filter in front that all incoming mail goes through.

I have the Proxmox mail filter in front of my Exchange. It works wonderfully well. No spam gets through.

There is also a Solaris sub reddit where people may want to grab some of the Sun hardware.

The E420's would be interesting since they can be quad cpu. Basically an Ultra 80 motherboard in a server chassis. I also had an E220R and E280R. Debian Linux is still being worked on so they could be used for that. Likely the NVRAM battery is dead.

I use my Nextcloud at my employment all the time. It's a great way to get scripts I have written deployed on client machines.

I use my Exchange server for all my alerting and subscribed mail lists. Plus I have the Proxmox mail filter in front of that which is really good at keeping the spam away.

I have self deployed Bitwarden and have come to depend on that very quickly. Pihole and Zabbix are other tools I use as well as having tftp and anon ftp servers up. So yes, my homelab is part of what allows me to work effectively.