could a beginner set up a mail server, and web hosting, using just the terminal?

> But single-board computers demand proprietary software to use GUIs,
> right? I think that they can be used freely with the command line (is
> this different from the terminal in a GUI system?).

Not true. RPi needs nonfree software to boot and run any software on
its ARM CPU, there are other computers listed at
https://www.fsf.org/resources/hw/single-board-computers that need it
only for 3d acceleration. A free distribution should be able to support
GUI on them without including such nonfree software. (There is a
partially related issue of no free distro supporting ARM yet.)

I don't believe a GUI would help in case of a mail server, it needs
mostly running some simple commands and editing text files.

> Do you think I could set up a mail server and hosting for a domain
> that I own, without stressing myself out?

Possibly yes, if you can spend enough time to learn it. Stress depends
on people, configuring a mail server was less stressing for me than
writing mail.

By the time you've paid for all the things you need in addition to the single board (e.g. SD card and power supply) you will have spent more than the cost of a second hand x86 / x86_64 netbook. Which will give you the added advantages of larger disk, WiFi (test it first) and a battery for a kind of UPS.

Then you can install and run those services and more from the command line by doing a Trisquel text mode install with just the console environment (+ SSH is an idea). Use the Ubuntu Server Guide for the appropriate release for your instructions.

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I'm in the process of writing a tutorial on setting up your own mail server using Trisquel. Hope to have it up on the Trisquel wiki soon.

I don't know about SBCs, though. They seem like a great idea, but we need one that will work in freedom.

Having said that, it is completely possible, and in fact advisable, to set up what you asked for without a GUI, so one of those boards that can be run in freedom except for the graphics acceleration would be suitable. You couldn't run Trisquel on it, because Trisquel doesn't provide binaries for ARM, but there may be ways to use other distributions freely.
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lloydsmart I am looking forward to try your tutorial out. thank you!