1) Mixture of high temperature and high Vcore will reduce the lifespan of the unit. The lower the Vcore the better, but this must be increased when OCing. Along with OCing the temperature at idle and load will be significantly higher.
2) i would not recommend temperatures over 72 degree Celsius. i think Intel and others on the forum stated this. My current build has both that CPU and heatsink and is currently clocked at 4.5 GHz with Vcore of 1.344 V. With the intelburntest my package temperature reaches 76 degree Celsius on maximum stress, which i think is okay as daily use will never push it that far.
You can find guides online as the 2500k is a very popular CPU to overclock.
Bottom line: which those temps and Vcore, stability test after OCing, and make sure you know how to clear CMOS.