for receipts and such paperless ngx is good. that won’t track your repairs or inform of you of likely maintenance problems, but that and a spread sheet sounds like a good start.

look into local dns servers if you want multiple machines to use your local domains if you only want a single windows or linux (and probably mac) computer to use the domain to access a specific local ip an entry in your etc/hosts file would be enough

https://github.com/docker-mailserver/docker-mailserver should be able to do it.

if i remember this in 6 weeks i’ll check the setup at work.