Off lease lenovo/HP/dell tiny/mini/micro. Keep the NAS for the storage, use the tiny/mini/micro as the media server. Anything with a 6th gen Intel and up will be decent on transcoding (use the igpu), 8th gen and up is better.
Personally I’d put proxmox on it, and run each service as an LXC, or for the little ones, one LXC for docker and throw the docker containers on there.
I have 7 tiny/mini/micros. 3 dell micro, 3 lenovo tiny, 1 HP mini (used to have 2 but one got replaced by a dell).
Powerhouses with low power draw. Highly recommended.
If you do build, I’d say an Intel arc GPU. The rest is just buying stuff that works well with Linux, which is most everything in terms of the basics.
KDE.
I won’t use gnome (I’ve mentioned elsewhere), and unsurprisingly I just dont like it either. The design choices are restrictive, the environment is oversimplified - its just not for me.
Ive used lots of DEs over the years, even fvwm95 (the original, its neat that some folks have updated it though), and at this point if its a desktop its getting KDE.