If you currently have an operational
C Band dish I recommend simply converting that dish to handle both C and Ku. Reason 1 is cost. You already have a motorized dish in place. To purchase a motorized dish along with everything required to install you will be easily approaching the costs involved in sinply replacing your feedhorn and a lnb for the big dish. I am assuming you do already have the unused coax in place.
Reason 2 is simple. A bigger dish traps more signal producing a higher quality picture and minimal rain fade.
Your big dish is most likely out of alignment so be prepared to align it properly. C band has a wide window that allows viewing of signal. Ku does not.