There are several different modalities of biofeedback equipment including EMG (Electromyograph) which measures muscle activity, Skin Temperature, Skin Conductance which measures changes in the amount of sweat on the skin, Respiration, and Heart Rate/Heart Rate Variability. EEG (Electroencephalography) or Brainwave Biofeedback is another modality which is usually dealt with separately and is also referred to as Neurofeedback.
You need to decide which of these modalities you want to work with. I suggest that you should have the capability of working with all of them especially for performing a psychophysiological stress profile or assessment. This is usually an initial session to get baseline readings on each modality at rest and under stress. I recommend at least 5 channels so that you can measure 5 different signals at the same time. You can do with fewer channels if you don’t plan to do stress profiles. Some systems will allow you to alternate different sensors on a two 2 channel system for example. You could have all of the modalities and just work with 1 or 2 at a time. I don’t suggest buying a system that only allows you to do one modality i.e. Temperature unless you have other equipment to do other modalities.
Training, Service, and Support
This may be the most important factor in making your choice. The equipment will not do anyone any good if you cannot use it. First you need to make sure that you can learn how to use the instruments that you are planning to purchase. Find out how you will learn how to use it. What kind of training manuals are available? Are there any tutorial videos, software, or workshops available? What is the process of getting service when you need it? Who can you call with questions? What country and time zone are they in? Some manufacturers are the only source of help and others use distributors who can also be a resource for help. Get these questions answered before making a biofeedback equipment purchase decision. If you do, it will help you have a much better experience.