Quality and reliability are very important when it comes to these monitors. You want them to work reliably and to provide an accurate depiction of what the camera is picking up and seeing at all times. You also have to consider whether you want a single monitor system, or a setup that contains dual rackmount monitors. You could always rent or lease several of the single monitors if you prefer.
Check to see the resolution of the monitors as well. The higher the resolution the better in most cases, but if you are merely using them to position your shots, a high resolution might not matter as much. Keep in mind that with the dual monitors on a single rack, the resolution is generally going to be lower than the single monitors.
How video preview monitors work
There are a number of different sizes and styles available when it comes to these monitors. You could choose a dual rackmount monitor system. These will generally have two monitors of the same size attached to a single rack. Alternatively, there are single monitors that might work better for you. One of the benefits of the single monitors is that they are generally larger, so if you need to have a bigger view of what the camera is seeing, they could be a good option.
You can connect cameras and playback devices to the monitor and then watch through them. They have many of the same controls as a computer monitor or even a television would have. One of the differences is that these monitors will also often have screen safe areas, and functions to help with focus.
Video Preview Monitor Manufacturers
Some of the most popular manufacturers making these types of monitors today include Blackmagic, Elvid, and Sony.