Acquiring and installing a used Aqualisa processor may be fine, but not advisable to do.  There are rubber components and plastic parts which with age loose flexibility and become brittle resulting in leaks.  These could be from the shower by not closing properly or to the housing which is “lethal” as often you cannot see the problem until damage has occurred.  If going for a used processor DO budget a service.  There is such on this site and e-Bay!