Which 4k Used Monitor?

Currently on a 1920x1200 monitor (I prefer the aspect ratio)
Budget is $300

I'd like a 4k monitor, 27inch (or 1 size higher).

What should I be looking for in terms of specs?

Is there a 'best' panel?
What about refresh rates?

Are there any things to watch out for apart from dead pixels?

Edit: some usage info:

  • not gaming
  • work use (ecxel, visio etc)
  • general browsing and media consumption