What kind of work/programs are you going to use the office pc for?
For general office use (ms office, email, etc)
* Core i3 CPU: Haswell or Ivy Bridge is fine
* ASUS, ASRock, or gigabyte mobo with video ports
* 8 GB ram
* 400ish watt 80 plus bronze cpu (corsair, antec, xfx, or even rosewill green)
* 120gb ssd if your budget allows for it.
* micro or mid atx case with decent airflow, depending on your needs.
If this is for work you should have sort of backup system in place, either on the pc or even better on a networked server.
If you dont have the needs for a dedicated server you can always just put a decent sized drive in a computer and share the drive and let everything backup to it.