5Ghz is quite high for a 2500k for day to day operation. You'll have to get lucky with a good chip to be able to hit that speed without needing a voltage level that will reduce the lifespan of your chip to only a few months. (Not to mention some serious cooling)
Most 5Ghz+ speeds for 2500k/2600k's are just for short term benchmarking, not running constantly. I believe one of the few I've seen that was for daily use actually used an open 100qt cooler with gallons of ice suspended in it to keep the temperature of the water under 10c, and was running at over 1.5v! (1.35 is a good top voltage to shoot for daily if you want your CPU to last years)
With as much as you'll have to spend on cooling anyways, you might want to go with the 2600k just for better binning that will hopefully let you use a little less voltage though.
As for the motherboard, it's a good board that shouldn't hinder you, but I think people generally use boards like the Sabertooth for 5Ghz+.
Is there a reason you need to hit 5Ghz? 4.6 is plenty for all games and will be much easier of your components.