I think that is a mistake. I have installed QM 8.5(1) without installing the Informix and it worked OK.
The 8.5(2) install guide does not say to install Informix.
The current informix driver is 3.70FC2 for 64 bit windows and 3.70TC1 for 32 bit windows. When Cisco install the UCCX historical reporting client that automatically installs the 3.0.0 client.
Graham