Just sailing? Single hard drive connected to what ever
Hosting stuff you care about? Some form of raid/zfs/whatever with at least 2 discs and a backup plan, also hooked up to what ever
That is the bare minimum. Buy used and expect your needs to change within a month/year.
Postfix and write a milter (mail filter), you can get them to interact a various points in the mail delivery.
I think most things can be accomplished within postfix