I have an old midi-tower standing around with everything inside but drives.

Is it stupid to just set up the drives as zfs inside the case and let my docker services run on the same machine (as long as there is enough RAM etc. of course)?

Or should I get another PC as application server?

I can afford enterprise-grade drives. It is rather that I have little to no clue about the reliability and failure-rates of different manufacturers/models.

And how different are these from consumer-grade ones? Is it cheaper to buy expensive drives once instead of multiple cheap ones one after another or do the quality differences not matter that much at all?

There is Mailcow. But simple is relative I guess cause you still gotta configure a lot around it to not end up on every spamlist out there