How much to pay?
If a printer is cheap the ink may not be so check this out before buying.
Laser or Inkjet?
Get a laser printer if printing more than 500 pages every month. Laser toner cartridges are designed for large volumes of printing, inkjets for less.
Mono or colour
It is always useful to have an option to print colour even if you print mainly in black. The key is to choose a printer that uses individual cartridges for black and colour so you can change each cartridge as it runs out saving money and ink.
Cartridges: Standard or High Capacity
Most laser printers designed for high levels of printing have standard capacity toners(normally 2-3000 page yield) or the high capacity (can be 6-8000 page yield) They cost more but are more cost effective especially as compatibles. HP denote the standard by an 'a' eg Q5949a and the high capacity by an 'x' eg Q5949x. Brother denote standard in number eg TN3130 the 3 referring to 3000 and in high capacity TN3170 the 7 referring to 7000 pages.
Multi-function printers (MFPs)
These printers can be great value for money with scanning, faxing and printing options saving space but not so good in a larger office if everyone needs to use them.
WiFi or Ethernet Networking
WiFi printers are definitely the way to go as you get rid of the wires in the office. Much easier to add printers to the network too.
Double sided printing
Printing on both sides of the paper automatically can save time and paper costs. They can however be slightly bulkier and take up more space in the office.
Automatic Document Feeder (ADF)
This is useful for large scale copying without manually feeding paper in thus saving time.
Large capacity paper trays allow for large workloads and save time adding more paper, pausing the print run which can waste ink and toner. Some printers can have a paper tray added so if you think you are going to printing alot it is worth looking into this. Also many printers can now print on as low as 75 gsm paper which will save on costs.
I hope this information was helpful for choosing an office printer.