Put the extra money on a good power supply, and some fans.
You want at least one intake and one outtake.
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.
Hey thanks a lot guys!
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?