Always put it at the top of your list. Other factors to be considered when choosing a machine have to do with the layout of the tennis courts. Often the most desirable playing courts are also the most ideal courts for ball machine use and this can cause a conflict. Consider building a storage shed near more remote courts. Run an electric line to courts that do not have them.
Portable battery powered machines are also a nice option for remote courts without power.
Battery powered machines have come a long way and with an auxiliary battery pack they can be a viable option for rental and lesson use. They are especially useful as a second machine because of their flexibility and ease of setup. Most ball machines today power the ball using counter rotating wheels to propel the ball in the same way that baseball pitching machines throw balls. If you are on a budget or want a second machine so you don't miss out on potential income because two players want to reserve a machine at the same time, air driven machines are an older technology that still retain the two important ingredients necessary for rental income. A low price combined with tremendous reliability.