It also depends on how much traffic/performance you expect out of the network.

Running on a single gigabit port will limit to a max performance of 1gbps.

Unless you have gigabit internet, it shouldn't be that big of a bottleneck, but it all depends on the use case...

Currently that's not possible unless you have business internet.

Basically ISP can mark whole IP ranges as residential, so any email sent from those is marked as spam no matter how you set it up...

It's cheaper to pay Google or similar $6 per month and avoid the hassle of locally hosted mail server.

A dual core will definitely not be enough, alas you need SMB multichannel working on both sides for this to work.