I’ve been using a TS140 with 4x WD Red 3TB drives for years as a FreeNAS server also running Plex. It’s a nice quiet, relatively low-power draw machine. (Mine idles at about 60 watts.) It only has two internal 3.5" drive bays. I mounted the other two in a 5.25 slot and in the external 3.5" slot. You can’t really get more than 5 drives into it unless you hack on the case. The motherboard only has 5 SATA connectors though, so you’d also need to add an HBA. Cooling might be an issue at that point. If you can make do with 4 or 5 drives total, then it’s decent.
This person transplanted his into another case to get more drives: