Thanks for the responses.
No, I won't use a dedicated GPU.
With the integrated graphics is more than enough for the initial setup of an Ubuntu Server, then I will manage it through SSH and maybe webmin.
I don't think that I will need 10 Gbit, the most demanding service that I run is the FTP and I'm the only person who use it, the FTP is only to upload backups of my Raspberry Pi once a day when nobody use the network at night.
There's no plans to make it accessible outside the house.
I don't think that DNS and Proxy servers needs tons of bandwidth, and DHCP usage is ridiculous.
In a daily basis we're 4 people in the house (and only we're all presents at night and weekends).
My internet connection is through 4G capped at 20 Mbps, I live in the middle of nowhere and I don't have any other option.
The "50 users/devices" is only once a year, at Christmas' Day when all my father's family comes to our house to spend that day all together.
It's not like I do LAN parties or that I own a CDN or something like that.
In that day most of the devices would be phones and tablets, always I can limit the bandwith of the WiFi mesh.
And the family knows that we have the crappy connection that we have.
And it's better that nobody complains about it because I'm very honest so I don't have any kind of problem to say something like the lines of: "Shut the fuck up, if you want a faster connection use your data, not mine". (I'm serious about it, sometimes my honesty is hurtful)
My actual server is an old PC, it has a 1st gen. Intel Core i5 and 4 gigs of RAM and works pretty well, but it wastes a good amount of space on my desk (and I don't want that).
With the current server running all those services I'm able to play Quake Champions and I don't notice any substantial lag increase playing through that old PC acting as a firewall.
If I have some minor problems, they are more related to the 4G connection than having a "poor performance" server with only 1 Gigabit interfaces.
Going back to my original post, where I'm having problems is finding a case which I can screw under the desk.
For CPU platform it will be easier with Intel, it has more ITX motherboards options than AMD, but as I said, if someone finds a good option of the red brand, it's welcomed.
I'll check out the Supermicro boards, and see what I can find.