You generally don’t need sudo to install flatpaks and actually pretty sure they advise against it.

I run my home media server ( and intel NUC i5, so nothing super powerful) running Ubuntu with CasaOS.

There’s tons of you tube videos to help with CasaOS for self hosting and not just the media side.

I think I only used the terminal to install CasaOS the rest is done from the web gui

My home media server is an old nuc mini pc i5 16Gb RAM with attached usb storage running on a Linux distro, runs Jellyfin and a few other applications for the household.

In short yes, an old pc will work fine.