Okay, so I've have a mini pc running 24/7 as a server. I have set up, among the other things, nextcloud and jellyfin. The pc has 1TB SSD built-in, which should be enough for basic use and storage (books, photos, personal papers). But I'm not becoming more greedy and thinking maybe 1Tb will not be enough, soon, if I want to store, say, blue ray movies, or copy of years of data that are in different clouds, or data lying in several portable drives.

NAS would be slightly overkill I'd say. I'm thinking of something more affordable. Could be another SSD (2TB?) to plug into a spare sata of the mini pc, or 2tb/4tb/8tb connected via usb3. What's recommended speed if hdd: 5400 or 7200rpm, or more? (I'm learning towards 5k4 as I heard it's less heat, thus less power consuming and lasts longer). Capacity & durability over performance (got a 3TB ssd in my gaming pc for that ;)).