Sure, yw :) There are also NAS cases for some of the SBCs, but I guess you can also go cheaper without a dedicated case and go with some icybox which allows you to connect some disks (jbod or RAID) via USB 3. So many possibilities!

I had some good experience with the pine64 boards!

It is open source if you are interested : https://github.com/MichMich/MagicMirror

In the end it is a scaffold on which you can install plugins which you like. The plugin community already provided many handy. Still it can be a challenge if you are not very "techy". Wish you the very best! You can do it :)

We recently set up a magic mirror (showing public transport connections/time/calendar/weather information) on a raspberry pi 3b. But it involved some more fiddling with electronics and software.

(Maybe an alternative would also be possible using small oled (128x64 pixel) screens)

Would be my suggestion if you are up for a challenge =)

We also used to host our own nextcloud, but decided to move it to hetzner as the pricing was unbeatable..

Else a pihole would also have been my suggestion. Maybe a Kodi mediacenter is also worth looking into.