Do you even need a stand-alone fax? Many multifunction printers include the ability to send and receive faxes. If you only need to transmit the occasional fax, this might be a better solution because you don't have to give up valuable desk space.
Consider the total cost. When considering the cost of a fax machine, the upfront price isn't the only factor to look at. Consumables such as toner cartridges, drums, thermal-transfer ribbons, etc., are often not inexpensive. In what's not all that much of a surprise, oftentimes faxes that have a higher upfront cost can use higher-capacity and/or lower-cost consumables than machines that look like a bargain initially.
Do you have a need for speed? Transmission speed ranges from about 15 seconds per page (for less expensive models like the Brother Fax-575) up to about three seconds per page (for fax machines with fast G3 modems, such as the Brother IntelliFax-2840).
How much memory do you want? Fax machines store incoming and outgoing pages in their onboard memory. Better faxes use that to allow the machine to do multiple tasks at once -- such as receiving a fax as one is being prepared to be sent. It also allows the machine to save an incoming fax if it runs out of paper, and then print the fax when the paper tray is reloaded. The most basic models have modest storage capacities -- about 25 pages -- while high-end fax machines can hold as many as 500 pages. Memory backup is another plus; it will hold the saved pages in memory should power temporarily go out.
Which features and extra functions do you need? Many fax machines include MFP-like extra functions such as copying, scanning and printing. Desirable fax-centric features include caller ID, auto redial, fax forwarding, polling, fax broadcasting and speed dialing.
Printing technology matters. Nearly all current fax machines print on plain office paper. Less expensive faxes generally use thermal-transfer printing technology, and printouts aren't as durable as inkjet or laser faxes. Laser fax machines usually print sharper text and stand up better to high-volume loads. They are the most expensive type of fax machines but are becoming more affordable. Inkjet fax machines are no longer being made but can still be found on occasion. Fax machines that print on thermal paper are, thankfully, pretty much relics.