Can I modify Informix server's configuration after I receive the system?

Yes. You have full access to and control of the system. Please make any modification to Informix, and iptables, the operation system, etc. as well, as you see fit.

Is Informix Warehouse Accelerator (IWA) included? How can I use IWA?

Yes, IWA is included. By default the product file is copied to the directory /opt/ibm/iwa; however, it is not configured. Please follow the instructions here to install and configure it.