i'd personally rent them from hire centres as and when you need them.
You wont need them in your treatment rooms as your client would be naked and get too cold. I know as a therapist you get all hot and sweaty but your moving about, and no matter how warm it is your clients body temp will be lower than yours.
I use large fans in my treatment rooms to cool them down during waxing etc, and In my reception and nail area a seperate aircon unit I hire.
It costs around £600 per room for air con to be fitted for professional use. Dont forget to take into account the costs of maintenance and PAT testing the Units too! An Air con unit must be maintained otherwise it can cause serious health risks.
Kate