Hmm..... no mention of ESET anyone?
Btw, my first post on this forum.
Avast or Avira. Why pay when you can get it for free.
My office uses Symantec Endpoint, but I think its not suitable or cost effective for SOHO. I'd used avast for so long I'm not sure about the current status of commercial antiviruses. From what I gathered from some recent reports, Bitdefender, Eset, Avast Paid Edition, Kaspersky will serve you good.
PS: AV-Comparatives Independent Tests of Anti-Virus Software » AV-Comparatives
Since its office premises, it would be better you went in for a paid AV to avoid any future hassles. We use quick heal Endpoint Security 5.3 for the last 1 year and thus far haven't faced any problems. But then you can always check user reviews & buy a software that you think can suit your office best.