VLC can do a lot of digital signage tasks on its own. I built out a raspberry pi for a startup that would just plug into a TV and run whatever videos were on the device, or slideshow through pics if no vids were there.
Add a touch of rsync or similar, and you're pretty much set for everything I can think of for digital signage.
You certainly can do this, but email is one of the few things I recommend not self hosting. My experience working at a spam filtering company really taught me how bad this can be.
Here's some of the reasons why I recommend against it:
Not getting an important email delivered because it came from a residential IP range and that alone puts you on several anti spam lists.
Not receiving an important email because some email admins put the blocklists on in both directions.
Sudden email issues because your IP changed. And now you have to re apply for all the blacklist removals again.
Internet/power goes offline at home, and some senders aren't configured to retry.
Get hit with a joe job attack and get gigabytes per hour of spam permanently, with no real way to stop it.
You may be sending and receiving too little email to actually remain in the good behavior cache for many spam filters.
One overeager cronjob or broken script and now you are on even more blacklists.
Misconfigure your server slightly and you are now a spam relay. Spammers scan the internet regularly for targets. You are not small enough to escape notice.
In case of house fire, what is your recovery plan? Have you tested it? Can you send "I'm okay" or insurance docs around if this system is permanently offline? How many weeks do you end up with zero email? How about people who depend on you as their provider?
Content disappearing is a big reason, but it's also great for channels that post b-roll or green screen videos/effects/etc if you like video editing.
Sure, you can grab them manually, or you can just have your folders of source materials available at the speeds of your local network, already stacked up and ready to work with.
Also, as someone in the infosec industry, a lot of conference talks and guides and useful videos get blown away whenever YouTube goes on a "hacking tutorial purge". This has already killed multiple channels full of useful guides.
Plus, YouTube has reencoded old videos and won't let you access the originals pre automated upscaling. YT is not a backup or an archival site. So, if that's something you want, you have to DIY it.