depends on your budget and need and whether you are okay with refurbished pc.  I have seen clients use old optiplex 780,760,755,745 running core 2 duo cpu, 8 GB ram and Windows 10 for internet and email and are happy with it.  for a bit newer (still old) and faster people pay on average $100-150 for optiplex 790 i5 or i7 second gen + 19 inch monitor package deal, which is pretty okay for general office work.  even newer pc has a newer look and USB 3.0 which may appeal too.
Which model is the best for a work office?
Optiplex, best model for a work office?