Could I do it on an old computer running Windows? You should be able to but doing it on linux could be easier
Or rould my modern home computer be the server Possibly but would not recommend it
The computer acting as a server would need to be on, and online all the time for me to access the files? This is correct and also why cloud hosts usually have different locations hosting the same thing to provide backup in case of power down or other problems
Would I be able to stream large files (video, audio, etc.) in real-time? Yes this should be possible but your homes internet or whereever youre keeping the server and its distance from where you access it will change speeds of doing that
Would I need to add an SSD if I use the old PC Not a need, i prefer to but i think opinions on this are mixed
If i want to have fun i use kde. If i want productivity and keeping devices running smoothly i go xfce. Lately its been more xfce