From my experiance with minecraft servers ryzen is the way to go.

I would recommend getting a used ryzen 5 5600g or ryzen 7 5700g i found some for less than 80€ on ebay. For the mainboard i would either get a cpu + mainboard combo or get it new as i haven’t found any non broken used boards. Pretty much anything is fine, just look out for the number of pcie slots and the lane distribution between them and if it supports lane bifurcation (you need this if you want to add m.2 expantion boards) if you want to add a hba or network card later on and that the board has 4 ram slots. Get at least 32gb of ram, 64gb is better and get them as 2 sticks, so you can upgrade later, ddr4 is cheap now. Storage wise I would reccommend 2 sata ssds as boot drives and 2 nvmes (if the mainboard supports it) for data.

So as an example (only 1 boot drive) with the prices ive found:

U: used (ebay); N: new

This mainboard isn’t itx!

Software wise you can try out debian, truenas or something else, but try to use zfs. Im personally using debian on zfs root running minecraft servers in docker containers with docker-compose but running lodestone (a web ui for mc servers) would also be an option. Running nextcloud in a container is also pretty eanew

I’ve experimented with ntfy, it works pretty well and is selfhostable. I don’t know if it natively support mqtt though.

I use both IONOS for domains and webhosting and a VPS from Host Unlimited, they also offer webhosting, but I haven’t tried it yet. I’m happy with both. Host unlimited apparently has a tier with unlimited mailboxes.

Volumes are horrible, how would I easily edit a config file of the programm running inside, if the container deosnt even start.

Bind mounts + ZFS datasets are the way to go.