And it really ought to generate binaural beat frequencies, where the difference in pitch between the left and right speakers is equal to the beat frequency in Hz. Find out how long the warranty lasts, what it covers, what escape clauses they give themselves, and what you have to pay to get a covered repair. The next tier of features are all very important, but not essential. I can imagine getting some use out of a machine that didn't have them. If a unit has preprogrammed sessions, there should be at least one beta, several each for different levels of alpha and theta, and at least one delta.
More variety is better, of course.
This lets you use mind altering tapes, like HPP, Paraliminal, or subliminal hypnosis tapes while running a mind machine session. Interestingly, small differences in frequency, 0.5 to 1.0 Hz, can make a big difference in the quality of the visuals. You may not think so, but if you have a portable unit, you'll find that you have more uses for it, as you take it places. Smaller and lighter is better. Different people will have different limits, but you should set yours.