CAT6 - avoid flat cable, and also avoid any low cost CCA.
Extenders - ensure whatever you go with has a good returns policy.
Extenders - be aware that some of the low cost HD over LAN single wire solutions will be compressing and processing the image, some will convert all 50Hz signals to 60Hz (which can look artificial when it's sports/anything which pans).
Joe
Fully Pre test any HDMI cable before you install it - I'd tend to avoid flat cable on anything longer than a couple of meters.