A modest overclock will have virtually no effect on a CPU's lifespan. It's only when you start to push things significantly that it will have a real impact.
A 2500k is normally 3.3 stock, 3.7 turbo. With virtually no effort you can disable turbo and set it to 3.8 (what the 2600k runs at on turbo by default) There you have 15% base performance without doing anything more than changing one number. (4.0 should be just as simply unless you are unlucky with what chip you get)
Virtually any $20 aftermarket cooler should let you reach 4.2 at minimum, and is a good base level to aim for if you want to play it very conservative. While coolers in the $30-40 range will usually get you at least 4.5.
It really has to do with what voltage your CPU will hit a certain speed at. Most people see 1.4v as the line where it starts to negatively affect lifespan, and depending on the CPU you get, you may hit 4.5, or 5.0 under that threshold. But I don't think I've read anyone saying that 4.2, let along 3.8, is detrimental to your CPU's lifespan.