If the cae is in good condition and you are happy with it, then there is no reason to replace it. It looks like a standard ATX case so it should be compatible a Z77 board. The stanchion placements haven't changed in the ATX standard so you're GTG on that front.
For a 10yr old case it's actually not that dusty. I'd do a thorough cleaning of the case and breadboard the new system prior to installing the new parts in the old case.
I will definitely be getting new fans and PSU and just be reusing the drivebays, optical drives, and case.
I will post in greater detail in October, as I understand it is a waste of time to speculate on what parts to get 14 weeks in advance.
Though as far as a PSU goes, I would like a "Seasonic X-Series X-1050 1050W Modular 80 Plus Gold" just to be on the safe side. I want to run dual graphics cards next year and do not want to be kicking myself in a few years when I want to upgrade... maybe 1250W?