Depends which route you wanna take:
- cheap and works out-of-the-box - entry level synology/qnap/asustore
- have some more room, cheap, expandable, want to tinker a bit: consider used/refurbished SFF/Tower, you can get some of them in very decent price, like HP Z230 (or other haswell+ CPU), slap truenas and there you go.
Personally I prefer the more DIY approach, as it doesn't lock me into hardware (e.g can diagnose/replace faulty parts instead of throwing away whole box) or software (synology actually use standard md/btrfs raid so it can be read by any modern linux). Although I was using Synology myself and was very happy about the software